We examined the effect of increased body core temperature (Tes) on the plasma arginine vasopressin concentration ([AVP]p) and thirst responses to increased plasma osmolality (Posm) induced by 3% NaCl infusion for 120 min in seven healthy humans. Tes was increased by immersion of the lower legs in 41 degrees C water in a 28 degrees C room (passive heating; HT). Immersion of the lower legs in 34.5 degrees C water on a separate day served as the control (thermoneutral; NT). The 120-min hypertonic saline infusion was initiated 30 min after the onset of leg immersion and was followed by a 30-min rehydration period. Tes in HT increased by 0.21 +/- 0.04 degree C before infusion and by 0.86 +/- 0.08 degree C at the end of infusion. The change in Tes in NT before and after the infusion was negligible. Posm was increased by 15.0 +/- 1.0 mosmol/kgH2O by infusion in both NT and HT. [AVP]p increased by 3.48 +/- 0.72 pg/ml in NT and by 7.59 +/- 1.02 pg/ml in HT. Thus the increase in [AVP]p at a given increase in Posm was markedly higher in HT than in NT. The plasma renin activity response to hypertonic saline infusion in both conditions was similar. Subjective thirst rating and cumulative water intake during rehydration were higher in HT than in NT.(ABSTRACT TRUNCATED AT 250 WORDS)