We investigated seasonal changes in hormonal and thermoregulatory responses. Eight volunteers were subjected to the experiment at four times of the year: around the vernal and autumnal equinoxes, and at the summer and winter solstices at latitude 35 degrees N. Plasma antidiuretic hormone (ADH), angiotensin II (ANG II), aldosterone (ALD) and plasma renin activity (PRA) were analyzed before and after water immersion. Seasonal changes in thermoregulatory responses were assessed by measuring core temperature and sweat rate during immersion of the leg in hot water (at 42 degrees C) for 30 min in a room maintained at 26 degrees C. The concentration of plasma ADH and ALD before water immersion was significantly higher in summer than in other seasons. The concentrations of ANG II and PRA did not show seasonal variations. Changes in tympanic temperature during water immersion showed significant differences between seasons, and were higher in winter than in other seasons. The sweat rate was significantly higher in summer than in other seasons. In summary, ADH and ALD concentrations displayed a seasonal rhythm with marked elevation in summer; this may be a compensative mechanism to prevent dehydration from increased sweat loss during summer due to heat acclimatization.