The zone of thermal comfort was determined during normoxia and hypoxia in 15 healthy normothermic young subjects. Subjects dressed only in shorts/shorts and bikini top donned a water-perfused suit and assumed a supine position on a bench. The ambient temperature was maintained at a mean (SD) of 25.7 (0.3) degrees C. The thermal comfort zone was determined by increasing the temperature of the water perfusing the suit from cool to warm. During the heating process, subjects were instructed to report when their perception of the thermal stimulus provided by the suit changed from unpleasant to pleasant, and again from pleasant to unpleasant. The boundaries of the thermal comfort zone were assumed to be the temperatures of the water perfusing the suit at the time the subjects reported a change in the affective component of their thermal perception. In normoxia, subjects inspired room air and in hypoxia a gas mixture containing 10% O(2) in N(2). Tympanic temperature was similar in the normoxia and hypoxia conditions (P>0.05). The average (SD) lower and upper limits of the thermal comfort zone were 30.5 (1.5) and 34.7 (3.3) degrees C, respectively, during normoxia, and 30.5 (1.7) and 35.1 (3.4) degrees C, respectively, during hypoxia. No significant differences were observed between the normoxia and hypoxia conditions (P>0.05). Also, no gender-related differences were observed in the characteristics of the thermal comfort zone. The results of the present study indicate that acute hypoxic exposure simulated in the present study does not affect the zone of thermal comfort in humans.