Heat acclimation has been suggested to either lower or have no effect on the rate of metabolism (M) elicited by muscular exercise. The purpose of the present investigation (Study I) was to examine the effect heat acclimation has on the M (W . kg-1 or VO2 in ml . kg-1 . min-1) elicited by muscular exercise. Two additional investigations were evaluated to determine if season (summer or winter) of year (Study II) and subject gender (Study III) further influence the effect heat acclimation has on M during exercise. Volunteers for Study I (n = 15 men), II (n = 8 men), and III (n = 10 men and 9 women) completed standardized treadmill walks in hot (40 degrees C, 30% rh or 49 degrees C, 20% rh) and cool (20 degrees C, 40% rh) environments immediately before and after heat acclimation. After heat acclimation, a lower M was observed for Study I (-4%; p less than 0.05), II (-2%; N.S.) and III (-3%; p = 0.06) in the hot environments. In addition, after heat acclimation a lower M was observed for Study I (-3%; p = 0.08), II (-5%; p less than 0.05) and III (-6%; p less than 0.05) in the cool environment. Season of year and subject gender did not have a significant effect on these results. These data indicate that heat acclimation does lower the M elicited by exercise. The observed percent decrease was lower in the hot (-3%) than cool (-5%) test environments.