Cardiorenal-endocrine responses to 3-h head-out immersion (HOI) (water temperature = 34.5 +/- 0.5 degrees C) were studied during day (0900-1400 h) and night (2300-0400 h) in six hydropenic male human subjects. Although HOI induced a reversible increase in urine flow in all subjects, the response was faster and greater in magnitude during the day compared with night (P less than 0.05). Na excretion and osmolal clearance (Cosm) also followed the identical response pattern as urine flow, and in fact, the HOI-induced diuresis was entirely accounted for by the increased Cosm. Endogenous creatinine clearance was not different between the day and the night and remained unchanged during HOI. Both plasma renin activity and aldosterone concentration and urinary aldosterone excretion were nearly twofold greater during the day compared with night before HOI but decreased to the same level during HOI in both daytime and the nighttime series (P less than 0.05). There was no correlation between the Na excretion rate and renin-aldosterone levels either before or during HOI. Plasma antidiuretic hormone (ADH) level was comparable between day and night before HOI and decreased to a similar level during HOI in both daytime and nighttime series (P less than 0.05 for nighttime HOI). Cardiac output increased from 3.3 1/min before HOI to 5-6 1/min during HOI without showing any significant circadian difference. Hematocrit, hemoglobin, and plasma concentrations remained unchanged under all conditions. It is concluded that the renal response to HOI is subject to nocturnal inhibition, which cannot be attributed to circadian differences in the degree of HOI-induced central blood pooling, renin-aldosterone, or ADH responses.