The authors tested the hypotheses that isoflurane anesthesia increases the threshold for sweating but minimally decreases the gain (sensitivity) or maximum intensity of this response and that thermoregulatory responses to hyperthermia are similar in anesthetized men and women. Sweating in response to core hyperthermia was studied in five men and five women during 0, 0.8, and 1.2% end-tidal isoflurane anesthesia. Thigh sweating was quantified by measuring gas flow, relative humidity, and temperature passing over a known surface area. The distal esophageal temperature triggering sweating was considered the sweating threshold, and gain was defined as the core temperature increment required to increase sweating rate from 25 to 75% of maximum observed intensity. The sweating threshold increased linearly with isoflurane concentration from 36.6 +/- 0.1 to 38.1 +/- 0.1 degrees C in the men and from 37.1 +/- 0.3 to 38.3 +/- 0.2 degrees C in the women. The thresholds were significantly higher in women than in men. Gain and maximum sweating intensities were similar at each anesthetic concentration and in men and women. These data indicate that isoflurane anesthesia significantly increases the threshold triggering thermoregulatory sweating but that gain and maximum sweating rate are relatively well preserved.