BACKGROUND:We assessed the effectiveness of SARS-CoV-2 vaccines against COVID-19-associated hospitalisation in paediatric patients in Hong Kong. METHODS:We conducted a test-negative design study in hospitalised children 1-17 years old admitted with recent-onset (≤5 days) acute respiratory illness in two large public hospitals in Hong Kong during successive SARS-CoV-2 epidemic waves between April 2022 and February 2024 dominated by the Omicron variant. We defined having been vaccinated as receipt of ≥1 SARS-CoV-2 vaccine dose within six months prior to hospitalisation, and for a sensitivity analysis, within twelve months of hospitalisation. Children who had only received one dose of vaccine and had no prior infection were excluded. We estimated VE against laboratory-confirmed SARS-CoV-2 infection separately for 1-4-year-olds and 5-17-year-olds, by conditional logistic regression stratified by calendar time (week) and adjusted for potential confounders. RESULTS:There were 6308 patients. The proportion of the vaccinated within six months of hospitalisation differed significantly between cases and controls in all strata of the assessed individual sociodemographic/health-related characteristics in the 5-17-year-olds, and in 1-4-year-olds who had a chronic medical condition. Overall, VE was moderate for 1-4-year-olds [41% (23-56%)] and for 5-17-year-olds [54% (10-66%)]. VE was higher in those who had received a mRNA vaccine: 87% (0-98%) and 82% (59-92%) compared with a lower VE for those who had received an inactivated vaccine: 39% (17-54%) and 30% (-19-58%), respectively for 1-4-year-olds and 5-17-year-olds although with a higher degree of imprecision in the VE for mRNA in 1-4-year-olds. In both age groups, VE was higher for those who had had no previous SARS-CoV-2 infection compared with for those who had had a previous infection. CONCLUSIONS:SARS-CoV-2 vaccines, particularly mRNA vaccines, provided substantial protection against paediatric COVID-19-associated hospitalisations in Hong Kong during two successive SARS-CoV-2 epidemic waves dominated by the Omicron variant.

BACKGROUND:Given concerns that influenza vaccine effectiveness (VE) might differ with repeated annual vaccination, we estimated VE against influenza-associated hospitalization and assessed repeat vaccination effects. METHODS:We analyzed a test-negative design study conducted in 3 Hong Kong hospitals (October 2015 - July 2025), excluding the 2020/21 and 2021/22 seasons due to the absence of influenza circulation during the COVID-19 pandemic. Polymerase chain reaction testing was used to identify influenza virus infections. We used conditional logistic regression to estimate influenza VE overall, by influenza type/subtype, and by influenza vaccination status in the preceding year (repeat vaccination status). RESULTS:We analyzed data on 34 237 children, among whom 5245 (15.3%) tested positive for influenza. VE against influenza-associated hospitalization was 57.2% (95% confidence interval [CI]: 52.3%, 61.6%), with subtype-specific VE estimates of 67.7% (95% CI: 61.8%, 72.7%) for A(H1N1)pdm09, 60.6% (95% CI: 50.8%, 68.5%) for influenza B, and 37.2% (95% CI: 24.7%, 47.6%) for A(H3N2). The overall ΔVE (repeated VE - current only VE) was -13.6% (95% CI: -33.2%, 3.2%), indicating lower VE among repeatedly vaccinated children. CONCLUSIONS:Current influenza vaccination programs provide substantial protection, but could be further improved by strategies that mitigate repeat vaccination effects, and further research is needed to identify such strategies.

Measuring waning in vaccine effectiveness (VE) is challenging due to potential depletion-of-susceptibles bias. Some SARS-CoV-2 studies excluded individuals with prior infection and adjusted for the probability of remaining uninfected. We applied this approach to assess waning influenza VE in Hong Kong during the 2012/2013 season. First, we estimated the infection risk for unvaccinated children using published serological and surveillance data. Next, we derived infection risk for vaccinated children, assuming VE against infection of 57%. Uncorrected VE from 14 to 270 days post-vaccination was estimated from hospitalized children. We calculated the rate of depletion of susceptibles given infection risk and VE corrected for depletion-of-susceptibles bias. Waning rates for uncorrected and bias-corrected VE were measured by comparing VE at day 270 versus day 14. Bias was assessed as the absolute difference between two waning rates in percentage points. Waning rate of uncorrected VE was overestimated by 5.9 percentage points or 1.3 percentage points when assessed up to day 120. Bias was substantial when assuming 80% unvaccinated, and all vaccinated children were initially uninfected, but minimal when these proportions were similar. The observed waning in 2012/2013 was unlikely due to depletion-of-susceptibles bias. Further studies across various conditions are needed to confirm our findings.

Influenza began circulating in Hong Kong in mid-December 2024. As of 12 March 2025, 1279 children were enrolled in a prospective test negative study for influenza vaccine effectiveness (VE). Among 209 influenzapositive cases, 71 % had influenza A(H1N1), 11 % had influenza A(H3N2), 3 % had influenza A with unknown subtype, and 15 % had influenza B. Among the test-negative controls, 53 % had received influenza vaccination since August 2024. End of the winter season VE estimates against hospitalization associated with influenza A (H1N1) was 64.8 % (95 % CI: 38.1 % to 80.3 %) and for influenza A(H3N2) was 59.9 % (95 % CI: -42.0 % to 89.6 %). These findings indicate that influenza vaccination provided protection against hospitalization for influenza A(H1N1) and A(H3N2) in children, highlighting the importance of continued vaccination efforts in Hong Kong.

BACKGROUND:IgE-mediated fish allergy has long been considered an umbrella term due to the high cross-reactivity of parvalbumin, the major fish allergen. Yet, clinical tolerance to certain fish highlights allergenicity differences. In this study, we sought to construct a fish allergenicity ladder and identify fish parvalbumin epitopes to improve the diagnosis of fish allergy. METHODS:Reported clinical history and the serum-specific IgE (sIgE) responses of 200 Chinese subjects with suspected fish allergy were collected and analyzed, while the relative parvalbumin content in different fish was measured for the construction of a fish allergenicity ladder. Double-blind placebo-controlled food challenge (DBPCFC) and open challenge against salmon, grass carp, and grouper were performed in 58 selected patients for validation of the ladder. Epitope mapping was performed by peptide array against parvalbumins of salmon (both β-1 and β-2), cod, grouper, and grass carp with sera from fish allergic (n = 11), partial fish tolerant (n = 12), and complete fish tolerant (n = 5) patients diagnosed based on oral food challenge outcome. RESULTS:The distribution pattern of reported history of fish allergy and tolerance, sIgE and molecular data, as well as their strong positive correlation led to the construction of a 3-step fish allergenicity ladder comprising: step 1 of the least allergenic fishes (tuna, halibut, salmon and cod), steps 2 of moderately allergenic fishes (herring and grouper) to step 3 of highly allergenic fishes (catfish, grass carp and tilapia). Epitope mapping revealed one epitope from grouper parvalbumin (AA64-78) for diagnosing general fish allergy and one epitopic region from salmon parvalbumin (AA19-33) as a biomarker of specific fish tolerance. Only epitope-specific IgE differentiated these patients but not sIgE to fish extract or parvalbumin. CONCLUSION:The fish ladder and epitopes discovery can precisely differentiate fish-allergic and tolerant subjects and guide fish reintroduction by stepping up the ladder, which innovates fish allergy care in the next millennium.

As intrinsic differences in humoral immune response to SARS-CoV-2 between children and adults remain unclear, we improved characterisation by defining the kinetics, specificity and function of antibodies to SARS-CoV-2 in children (n = 146, aged 9.4 ± 4.8 years with n = 257 samples) compared to adults (n = 85, aged 39.5 ± 15.2 years with n = 122 samples). We used plasma samples from an infection and vaccination-naive cohort study with RT-PCR confirmed ancestral B.1* SARS-CoV-2 virus infection with asymptomatic or mild disease, collected in Hong Kong between March to December 2020, from acute (0-14 days post infection) to convalescent (15-206 days) timepoints. Children had significantly lower primary antibody responses against SARS-CoV-2 proteins overall, leading to a less isotype switched response. While children had lower OC43 Spike and SARS-CoV-2 S2 IgG and avidity than adults, they exhibited higher avidities for SARS-CoV-2 whole Spike and Nucleocapsid, and higher levels of Spike FcγR-binding antibodies. Adults' SARS-CoV-2 antibody responses could be derived from high avidity pre-existing cross-reactive common cold coronavirus B cell responses, whilst children appear to generate a de novo SARS-CoV-2- specific Spike and Nucleocapsid IgG with robust Fc receptor (FcR) binding ability and high avidity at a higher proportion than adults, thus their responses are more targeted and functional for SARS-CoV-2.

OBJECTIVE:To develop a YouthFit Index, a comprehensive and easily interpretable measure of youth health-related quality of life (HRQoL). METHOD:A repeated cross-sectional design was employed to develop and validate the YouthFIt index using the Pediatric Quality of Life Inventory (PedsQL). Activity level and sleep data were collected by a wearable device, Actigraph. Questionnaires were used to gather demographic information and PedsQL data. Multiple-factor analysis of mixed data was employed to identify principal components (PCs) of the modifiable lifestyle variables. The YouthFit Index was calculated by summing the weighted PCs. Logistic regression models were used to assess the odds ratios for risks associated with the YouthFIt Index. RESULTS:A total of 1,867 students were recruited for the study. The findings showed that having YouthFit Index scores greater than 5 was significantly associated with a lower risk of identifying with special health care needs. Compared to the group with YouthFit Index scores below or equal to 5, the group with scores above the cut-off demonstrated a significant 43 % reduction in the risk of PedsQL total score. The results also revealed that increased sleep duration and physical activity are associated with better HRQoL, while longer sedentary periods and later bedtimes are linked to worse HRQoL. CONCLUSIONS:The YouthFit Index is a user-friendly tool for assessing the HRQoL of Hong Kong Chinese children between the ages of 6 and 17. It can help raise awareness about healthy quality of life and promote health and well-being among children in Hong Kong.

BACKGROUND: Seafood is a common cause of food allergyand anaphylaxis, but there are limited published real-world datadescribing the clinical presentation offish and shellfish allergies. OBJECTIVE: This study aimed to examine the clinicalcharacteristics, immunological profile, and tolerance pattern tofish, crustaceans, and mollusks infish-allergic individuals. METHODS: Patients presenting with IgE-mediatedfish allergybetween 2016 and 2021 were recruited. A comprehensivesensitization profile including specific IgE and skin prick test tovariousfish and shellfish species and a detailed clinical historyincluding individuals'recent seafood consumption wereevaluated. RESULTS: A total of 249fish-allergic individuals (aged 4.2-5.8years) were recruited from 6 allergy clinics in Hong Kong, andthey had experienced theirfish-allergic reaction 2.2-3.4 yearsbefore enrollment. Seventy-five subjects (30%) reacted to eithergrass carp, salmon, grouper, or cod in oral food challenges. Weidentified an IgE sensitization gradient that corresponded to thelevel ofb-parvalbumin infish. In total, 40% offish-allergic in-dividuals reported tolerance to 1 or more types offish, morecommonly tofish with a lowerb-parvalbumin level such as tunaand salmon, compared withb-parvalbumin-richfish such ascatfish and grass carp. Despitefish and shellfish cosensitization,41% of individuals reported tolerance to crustaceans, mollusks,or both, whereas shellfish avoidance occurred in half of thefish-allergic individuals, of whom 33% lacked shellfish sensitization. CONCLUSIONS: Fish allergy commonly presents in earlychildhood. A considerable proportion offish-allergic patients areselectively tolerant to certainfish, typically those with lowerlevels ofb-parvalbumin. There is an unmet need to promoteprecision medicine for seafood allergies. (c) 2023 The Authors. Published by Elsevier Inc. on behalf of the American Academy ofAllergy, Asthma & Immunology. This is an open access articleunder the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Vaccine-related myocarditis associated with the BNT162b2 vaccine is a rare complication, with a higher risk observed in male adolescents. However, the contribution of genetic factors to this condition remains uncertain. In this study, we conducted a comprehensive genetic association analysis in a cohort of 43 Hong Kong Chinese adolescents who were diagnosed with myocarditis shortly after receiving the BNT162b2 mRNA COVID-19 vaccine. A comparison of whole-genome sequencing data was performed between the confirmed myocarditis cases and a control group of 481 healthy individuals. To narrow down potential genomic regions of interest, we employed a novel clustering approach called ClusterAnalyzer, which prioritised 2,182 genomic regions overlapping with 1,499 genes for further investigation. Our pathway analysis revealed significant enrichment of these genes in functions related to cardiac conduction, ion channel activity, plasma membrane adhesion, and axonogenesis. These findings suggest a potential genetic predisposition in these specific functional areas that may contribute to the observed side effect of the vaccine. Nevertheless, further validation through larger-scale studies is imperative to confirm these findings. Given the increasing prominence of mRNA vaccines as a promising strategy for disease prevention and treatment, understanding the genetic factors associated with vaccine-related myocarditis assumes paramount importance. Our study provides valuable insights that significantly advance our understanding in this regard and serve as a valuable foundation for future research endeavours in this field.

Objective To determine whether health-related physical fitness and body mass index (BMI) status differed before and after school closure from the COVID-19 pandemic in a population-based cohort of Hong Kong primary school-children. Study design We examined the BMI z score, BMI status, and physical fitness z scores including (i) upper limb muscle strength, (ii) 1-minute sit-up test, (iii) sit-and-reach test, and (iv) endurance run tests, among 3 epochs: pre-pandemic (September 2018-August 2019), before school closure (September 2019-January 2020), and partial school reopening (September 2021-August 2022), using a repeated cross-sectional approach. Results A total of 137 752 primary schoolchildren aged 6-12 years were recruited over 3 academic years. Obesity increased significantly from 25.9% in 2018/19 to 31.0% in 2021/22, while underweight increased slightly from 6.1% to 6.5%. All tested parameters were adversely affected by the pandemic. The negative trend over time was far more pronounced in all 4 physical fitness scores in the underweight group, although performance in handgrip strength had no significance between 2018/19 and 2021/22. Conclusions Schoolchildren who are both underweight and overweight/obese are vulnerable to adverse changes in physical fitness during the COVID-19 pandemic. To eliminate the negative health and fitness outcomes, it is urgent to develop strategies for assisting schoolchildren in achieving a healthy weight, especially in the post-pandemic era. (J Pediatr 2024;264:113729).

A test negative study was carried out from 13 June through to 15 November 2023 enrolling 3183 children hospitalized with acute respiratory illness in Hong Kong. Influenza A and B viruses were detected in 528 (16.6%) children, among which 419 (79.4%) were influenza A(H3N2). The overall vaccine effectiveness against hospitalization associated with any influenza virus infection was estimated as 22.4% (95% CI: -11.7%, 46.1%), and against influenza A(H3N2) specifically was 14.3% (95% CI: -29.2%, 43.2%). Despite the moderate to low VE estimated here, which could be a result of waning immunity and antigenic drift, influenza vaccination remains an important approach to reduce the impact of influenza in children.

We conducted a test negative study from November 2023 to June 2024, enrolling 4,367 children hospitalized with acute respiratory illness in Hong Kong. Among the children who tested negative for influenza virus and SARS-CoV-2, 56.8 % had received influenza vaccination. Between November 2023 and March 2024, influenza A(H3N2) predominated and the VE against influenza A(H3N2) was estimated as 55 % (95 % CI: 29.6 %, 71.8 %). VE point estimates were higher for younger children than older children. In February to June 2024 influenza A(H1N1) predominated and VE against influenza A(H1N1) was 54 % (95 % CI: 33 %, 69 %) during this period. Influenza B/Victoria circulated at low intensity throughout the 2023/24 season and VE against influenza B was 66 % (95 % CI: 42 %, 80 %). Since its introduction in 2018/19 the school-based influenza vaccination program has substantially increased vaccine uptake in children in Hong Kong and prevented influenza-associated hospitalizations.

This study aimed to establish sex- and age-specific reference values for motor performance (MP) in Hong Kong preschoolers aged 3–5 years old and examine the relationship between MP and BMI status. A cross-sectional study was conducted among 5,579 preschoolers in Hong Kong. Three MP tests were administered, and height and weight information were collected. GAMLSS was used to compute the normative values of the motor tests. Boys outperformed girls in activities requiring muscle strength and power, while girls outperformed boys in activities requiring balance and coordination. The MP scores increased with age for both overarm beanbag throw and standing long jump for both sexes, while the one-leg balance scores showed larger differences between P 50 and P 95 in older preschoolers. Children with excessive weight performed worse in standing long jump and one-leg balance compared to their healthy weight peers. This study provides valuable information on the MP of preschoolers in Hong Kong, including sex- and age-specific reference values and the association between BMI status and MP scores. These findings can serve as a reference for future studies and clinical practice and highlight the importance of promoting motor skill development in preschoolers, particularly those who are overweight or obese.

INTRODUCTION:Various cutaneous manifestations have been reported as symptoms of coronavirus disease 2019 (COVID-19), which may facilitate early clinical diagnosis and management. This study explored the incidence of cutaneous manifestations among hospitalised patients with COVID-19 and investigated its relationships with viral load, co-morbidities, and outcomes.METHODS:This retrospective study included adult patients admitted to a tertiary hospital for COVID-19 from July to September 2020. Clinical information, co-morbidities, viral load (cycle threshold [Ct] value), and outcomes were analysed.RESULTS:In total, 219 patients with confirmed COVID-19 were included. Twenty patients presented with new onset of rash. The incidence of new rash was 9.1% (95% confidence interval=6.25%-14.4%). The most common manifestations were maculopapular exanthem (n=6, 42.9%, median Ct value: 24.8), followed by livedo reticularis (n=4, 28.6%, median Ct value: 21.3), varicella-like lesions (n=2, 14.3%, median Ct value: 19.3), urticaria (n=1, 7.1%, median Ct value: 14.4), and acral chilblain and petechiae (n=1, 7.1%, median Ct value: 33.1). The median Ct values for patients with and without rash were 22.9 and 24.1, respectively (P=0.58). There were no significant differences in mortality or hospital stay between patients with and without rash. Patients with rash were more likely to display fever on admission (P<0.01). Regardless of cutaneous manifestations, patients with older age, hypertension, and chronic kidney disease stage ≥3 had significantly higher viral load and mortality (P<0.05).CONCLUSION:This study revealed no associations between cutaneous manifestation and viral load or clinical outcomes. Older patients with multiple co-morbidities have risks of high viral load and mortality; they should be closely monitored.

Background The current diagnostics of fish allergy lack sufficient accuracy such that more reliable tests such as component-resolved diagnosis (CRD) are urgently needed. This study aimed at identifying fish allergens of salmon and grass carp and evaluating the sensitization pattern towards the identified allergens in fish allergic subjects from two distinct populations in Asia. Methods One hundred and three fish allergic subjects were recruited from Hong Kong (67 subjects) and Japan (46 subjects). Western blot and mass spectrometry were used to identify allergens from salmon and grass carp. Fish allergens were purified and tested against 96 sera on ELISA to analyze patients’ sensitization pattern. The protein profiles of salmon meat prepared under different cooking methods until core temperature reached 80°C were evaluated by SDS-PAGE and mass spectrometry. Results Three common allergens between salmon and grass carp, namely enolase, glycerldehyde-3-phosphate dehydrogenase (GAPDH) and parvalbumin, and two salmon-specific allergens collagen and aldolase were identified. Parvalbumin was the major allergen for both fishes showing an overall sensitization rate of 74.7%, followed by collagen (38.9%), aldolase (38.5%) and enolase (17.8%). Japanese subjects showed more diverse allergen sensitization pattern and more frequent IgE-binding to heat-labile salmon allergens. Compared with steaming and boiling, cooking by baking and frying retained more fish proteins inclusive of heat-labile allergens. Conclusions Fish allergic patients from different Asian populations show varying fish allergen sensitization profiles. The relevant extracts and components for diagnosis are population-dependent but parvalbumin and collagen are important biomarkers. Cooking methods modify allergen composition of salmon and appear to influence patients’ allergic manifestations.

A growing body of research suggests an association between attention deficit hyperactivity disorder (ADHD) and allergic disorders, but little work has been done to explore the role of external factors such as parental smoking at home in the development of comorbid ADHD and allergic disorders. This study aimed to examine the association between allergic diseases and ADHD adjusted for exposure to parental smoking at home in early adolescents. We recruited 250 male (41.7%) and 350 female (58.3%) adolescents (mean [SD] age, 13.29 [0.52] years) via chain-referral sampling. Their ADHD symptoms were assessed by the parent proxy-report version of the Chinese Strengths and Weaknesses of Attention-Deficit/Hyperactivity-symptoms and Normal-behaviours (SWAN) rating scale. Data on the participants’ history of clinician-diagnosed allergic diseases, family socio-demographics, and parental smoking habit were collected using a parent-completed questionnaire. Regression analyses were performed to examine the associations of interest. The levels of ADHD symptoms were comparable between allergic and non-allergic participants after controlling for child and family demographics and parental smoking at home. Notably, the risk of probable ADHD was particularly high in participants with food allergies (odd ratio = 4.51, p = 0.011) but not in those with allergic rhinitis after adjusting for parental smoking at home. Our findings suggest that second-hand smoke exposure at home is a potential risk factor underlying the link between ADHD and allergic diseases. Current management guidelines should emphasize the importance of early identification and cessation of tobacco smoke exposure for prevention of comorbidity of ADHD and allergic disorders. Clinical Trial Registration (if any): NA.

In March-June 2023, we conducted a test-negative study in 1671 children who were hospitalized with acute respiratory illness in Hong Kong. Two hundred and eighty-six children (17.2%) were tested positive for influenza virus including 188 with A(H1N1). We estimated influenza vaccine effectiveness against influenza-associated hospitalization as 69.6% (95% confidence interval: 49.3%, 81.7%).

In June 2020, a previously healthy 7-year-old boy presented with a 1-week history of persistent fever.He had an unremarkable medical history, and had gone hiking a week before the onset of his fever.He was initially treated for a presumed viral illness but his condition worsened over the subsequent 2 days.His fever became high and fluctuating, with a peak of 39°C, and he developed dyspnoea without cough.He had no headache or body aches.Physical examination revealed crepitations with diminished breath sounds over both lung fields and tachypnoea with a respiratory rate of 30 per minute.Cervical lymphadenopathy and hepatomegaly were present.There was no eschar.He had distributive shock with hypotension (80/40 mm Hg) and tachycardia (150 beats per minute) and a norepinephrine infusion was commenced.Within one day, type I respiratory failure became evident with increasing oxygen requirement from room air to FiO 2 0.4 and continuous positive airway pressure of 6 cmH 2 O to maintain oxygen saturation above 90%.The PaO 2 :FiO 2 ratio was 232.5 mm Hg.Plain radiograph of the chest revealed bilateral opacities and peribronchial thickening (Fig) .Echocardiography and lung ultrasound confirmed normal heart function and the absence of pleural effusion.By definition, the boy was diagnosed with paediatric acute respiratory distress syndrome (PARDS: an acute lung injury occurring within 7 days of a known clinical insult, with acute hypoxaemia of PaO 2 :FiO 2 ratio ≤300 when the child is on noninvasive ventilation, and new infiltrates consistent with acute pulmonary parenchymal disease on chest radiograph, which cannot be explained by acute left ventricular heart failure or fluid overload), where he had progressive respiratory failure and bilateral diffuse infiltration on chest radiography. 1 Further blood tests and bone marrow findings met the diagnostic criteria for haemophagocytic lymphohistiocytosis (HLH: a life-threatening clinical syndrome of systemic hyperinflammation and progressive immune-mediated organ damage

A dose of oxytocin (50 ng i.c.v.) that induces penile erection and yawning, increased the concentration of NO2- from 0.98 +/- 0.29 to 4.2 +/- 0.79 microM and of NO3- from 5.6 +/- 0.33 to 12.03 +/- 0.99 microM in the dialysate from the paraventricular nucleus of the hypothalamus of male rats, as measured by in vivo microdialysis. NO2- concentration was also increased by [Thr4, Gly7]-oxytocin (100 ng i.c.v. and oxytocin(8) (1 microgram i.c.v.) which also induced penile erection and yawning, but not by oxytocin(1-6) (1 microgram i.c.v.) or oxytocin (7-9) 1 microgram i.c.v.), which were unable to induce these behavioral responses. The oxytocin effect on NO2 concentration, penile erection and yawning was prevented by the oxytocin receptor antagonist. d(CH2)5,Tyr(Me)-Orn8-vasotocin (1 microgram i.e.v.) or by the nitric oxide synthase inhibitor, NG-nitro-1-arginine methyl ester (200 micrograms i.c.v.), but not by the dopamine receptor antagonist, haloperidol (0.5 mg/kg i.p.). The nitric oxide scavenger, hemoglobin (200 micrograms i.c.v.), prevented oxytocin-induced NO2- concentration increase, but was unable to prevent penile erection and yawning. Methylene blue (300 micrograms i.c.v.) an inhibitor of guanylate cyclase, was ineffective on oxytocin-induced NO2- concentration increase, but prevented the behavioral responses. The results suggest that oxytocin induces penile erection and yawning by increasing nitric oxide synthase activity in the cell bodies of oxytocinergic neurons projecting to extra-hypothalamic brain areas and mediating the behavioral responses.