Introduction: Recent studies have established that growth and development of follicles to the antral stage is possible within cryopreserved human ovarian tissue following autologous and heterologous transplantation. Although cryopreservation appears to have preserved the capacity for follicular development to the antral stage, evidence of ovulation is limited to indirect observations of a single follicle while the majority of antral follicles appear to be destined for atresia. The aim of this study was to assess the response of antral follicles within xenografts of cryopreserved ovarian tissue to administration of exogeneous hCG. Methods: Ovarian tissue was donated from 2 patients at risk of loss of fertility. Thin slices of ovarian cortex were cryopreserved as previously described. Fresh and cryopreserved tissue pieces were transplanted into female immunodeficient (SCID) mice > 20 weeks old. A single piece (0.5 x 1 x 1mm) of ovarian cortex was placed under the capsule of each kidney and mice were oophorectomised bilaterally. Intraperitoneal injections of gonadotrophin (1 IU/ml rec FSH) or saline were given on every second day. At ≥ 36 weeks post transplantation mice received an intraperitoneal injection of 20 IU of hCG. Kidneys were removed 15 and 30 hours after hCG, examined for gross morphological changes in the antral follicular appearance and fixed for histology. Results: The frequency and size of antral follicles observed were consistent with our previous results. No obvious difference was observed in gross morphology or histology between fresh or frozen tissue following transplantation or with administration of gonadotropin. On examination, the transition from a transparent to an opaque antral cavity was apparent 15 hours after hCG in all follicles. In these follicles, histological examination identified only minor changes consisting of loss of continuity in the inner layers of mural granulosa cells which slough into the cavity, and the presence of an occasional luteinized granulosa cell situated adjacent to these sloughing granulosa cells. Low numbers of luteinized cells were also observed amongst the stromal cells of the ovarian cortex. In contrast, histological examination of follicles fixed 30 hours after hCG revealed large areas of ovarian cortex, in close prioximity to the antral follicles, which contained luteinized cells. Luteinized cells and red blood cells were now present within both the follicle wall and the antral cavity. The granulosa cell layers had lost their organisation and appeared to be reduced in numbers. Similarly, the cumulus cells were now interspersed by an amorphous matrix and the germinal vesicle was absent or undergoing breakdown within the oocyte. Conclusion: These observations indicate a responsiveness of the cryopreserved ovarian tissue and follicles to hCG, suggesting preservation of the ability to undergo periovulatory changes, under appropriate conditions.