We investigated the seasonal variation of sweating response during exercise. Four adult heaithy men repeated a moderate bicycle exercise (60 watts) in a climatic chamber of an ambient temperature of 30瀉 (relative humidity, 45%) in winter, spring, summer, and fall. In summer, sweat rate immediately increased as soon as the exercise started, whereas in winter in a few minutes. The mean sweat rate during exercise was significantly different between winter and summer. The transient reduction of the T_SK was observed at the beginning of the exercise in winter. The T_SK decreased in proportion to increasing of sweat rate in each season. Significantly negative correlation were found between sweat rate and the rate of change of T_SK during exercise in each season. The slope and intercept of regression line were significantly different between winter and summer. The index of sweating was made available for the relative value, changing rate against annual mean value of total sweat loss (ΔSR, %). The relative value rather than the absolute value (i.e., expressed as g・m^<-2>・h^<-1>) corrected well with skin temperature. It is suggested that the present results may reflect adapted changes in the thermoregulatory mechanisms to seasonal acclimatization. Moreover, the fall in skin temperature during exercise may be not due to increased evaporative cooling, but may be the result of vasoconstriction porbably caused by non-thermal factors.