Five distance runners (H groups) performed a 60 min bicycle exercise at a load of 60--70% VO2max in a moderately hot environment (Ta: 33.5 degree C, 60% RH). Following a period of heat acclimation with bench-stepping at a load equal to about 25--30% VO2max, in a hot environment (Ta: 45--50 degree C, 30--40% RH) for 9 days, the work test was repeated. Two control subjects (R) performed the same work tests with no heat acclimation. Heat acclimation increased performance time. Rectal temperature, mean skin temperature, heart rate, and Na+ concentrations in sweat were lower in H and, with one exception, sweat rate was higher after heat acclimation. All H subjects demonstrated that the linear relationship between sweat rate and rectal temperature was shifted to a lower temperature (threshold shift). This shift correlated with a lowering of resting rectal temperature. The magnitude of the reduction in those two temperatures due to heat acclimation was identical. The observed improvement of work performance in moderate heat following heat acclimation to a higher temperature is attributed to a more efficient thermoregulatory mechanism.