The effect of physical training on the time course of sweat rate in women has been examined. Physically untrained and trained young female subjects pedaled a bicycle ergometer at work rates of 483 and 981 kgm.min-1, for 2 h in summer and winter in an ambient environment of 30 degrees C db and 60% rh. The trained women also worked at 1,070 kgm.min-1 and the untrained at 391 kgm.min-1. Rectal temperature was measured, and capsule sweat samples were collected from the back every 5 min. Sweating was initiated more rapidly in the trained group than in the untrained group. The trained group working at a load of 981 kgm.min-1 exhibited a progressive decrease in sweat rate. This was not observed at a work load of 483 kgm.min-1. Hidromeiosis was rarely seen in the untrained group. However, in the untrained women who underwent 60 days of physical training, initiation of sweating occurred more quickly and hidromeiosis was observed. It was concluded that previous physical training improved women's capacity for useful sweating during exercise in a hot environment.