Nine young men who had suffered from heatstroke on previous occasions (heat-intolerant subjects) and 10 young volunteers (control subjects) were examined to determine their physiologic responses to exercise in temperate (23 degrees C) and hot environments (40 degrees C). The tests included an orthostatic test, work loads of 40 W and 80 W, and oxygen consumption (Vo2) determination. Although all the control subjects completed the exercise under severe heat load (3 h), none of the heat-intolerant subjects succeeded in completing this test due to high rectal temperatures and high heart rates. Sweat rates were similar in both groups, with Vo2 slightly higher in the control subjects. Orthostatic responses were similar in each group. The results suggest that inefficient thermoregulation, possibly due to decreased heat conductance from core to periphery, contributes to heat intolerance in former heatstroke patients.