We evaluated the hypothesis that different rates of metabolic heat production between sexes, during exercise at the same percentage of maximum oxygen consumption [VO2 max] give proportional differences in evaporative heat loss. Seven males and seven females, exercised at 41.3 +/- 2.7% VO2 Max for 60-min at 40 degrees C and 30% relative humidity. Whole-body direct air calorimetry measured rate of whole-body evaporative heat loss (H e) while metabolic heat production (M - W) was measured by indirect calorimetry. M -W was greater in males (243 +/- 18 W m(-2)) relative to females (201 +/- 4 W m(-2)) (P <or= 0.05) throughout exercise. This was paralleled by a greater H e at end-exercise in males (207 +/- 51 W m(-2)) relative to females (180 +/- 3 W m(-2)) (P <or= 0.05). Differences in metabolic heat production between sexes during exercise at a fixed percentage of VO2 max give differences in evaporative heat loss. To compare thermoregulatory function between sexes, differences in metabolic heat production must therefore be accounted for.