In seven long-distance runners (42 km or more) the thermoregulatory responses to acute external cooling and heating, under resting conditions, were recorded and compared with those in physically untrained controls. Sweating as well as shivering thresholds were significantly decreased in the runners when compared either in terms of mean body temperature (Tb) or esophageal temperature (Tes); Tb and Tes were reduced in the runners at rest under thermoneutral conditions. Moreover, cold sensation in the runners occurred at lower Tb. The runners thus behaved as if the "set point" of their thermoregulatory system had been reset to a lower level. As for the sweating threshold, the shift is quantitatively comparable to that found in heat adaptation. The described modifications in long-distance runners would prolong the time period until a dangerous body temperature-one of the important limiting factors in physical endurance-is reached during heavy exercise.