Five prepubertal females and five college women, matched for aerobic power, walked on a treadmill at approximately 30% VO2 max for two 50-min periods in three environments: 1) 28 degrees C, 45% rh, 2) 35 degrees C, 65% rh, and 3) 48 degrees C, 10% rh. In the mild heat (28 degrees C) both groups were able to work 100 min with no discomfort. At 35 and 48 degrees C tolerance time for the prepubertal subjects averaged 84.4 and 37.0 min, respectively; for adults, 100 and 75.0 min. At all temperatures the girls had higher heart rates and a lower stroke index, and finished the walks with a higher rectal temperature. There were no differences between groups in cardiac index, mean skin temperature, forearm blood flow, or percent loss in body weight. The proportion of the thermal load dissipated by the two groups was similar but the route for heat transfer was related to the BSA/wt ratio and environmental conditions. Marked circulatory instability was a primary factor in the lower tolerance of the prepubertal girls to work in the heat probably due to a shift in blood volume from the central to the peripheral circulation.