To determine the relation of rectal temperature (Tre) to Vo2max and heat tolerance, eight untrained, eight trained and five heat acclimated subjects (respective means +/- SE for Vo2max in ml/kg.min of 37.5 +/- 1.6; 55.7 +/- 1.5; and 54.5 +/- 3.2) were tested in 3 conditions: 60 min of exercise at a fixed load of 35 W at room temperature of 23 degrees C; 60 min of exercise at 35% Vo2max also at 23 degrees C, and 3-hr of exercise in heat (40 degrees C DB, 30 degrees C WB). The heat-acclimated group showed the best heat tolerance, while the untrained group showed the poorest responses in heat. Exercise at 35 W resulted in higher heart rates shown by the untrained, compared with the other subjects, while equilibrium Tre were 37.6, 37.9, and 38.2 degrees C, in the heat-acclimated, trained and untrained groups, respectively, with corresponding differences for resting Tre (36.7, 36.9, and 37.1 degrees C). During exercise at 35% Vo2max, the heat-acclimated group showed lower Tre than the trained group despite working at the same relative loads. Tre during exercise at 35 W at 23 degrees C correlated r = -70 with Vo2max and r = 0.80 with Tre during exercise in heat. These results show that Vo2max accounts for only part of the variability which determines the level of Tre in cool conditions with heat acclimatization accounting for the remainder of this relationship.