Renal and hemocynamic responses to central vascular volume expansion induced by 4 hr of continuous lower body positive air pressure (LBPP) were examined in conscious, chair-restrained squirrel monkeys in a light/dark (12:12) cycle. LBPP (30 mmHg) during both day (1200--1600) and night (0000--0400) induced similar 4 cmH2O stable increases in central venous pressure (P < 0.001), rises in heart rate of 25 beat/min (P < 0.001), and small transient elevations in mean arterial blood pressure. In contrast, while daytime LBPP induced a significant increase in urine flow (V) from 2.12 +/- 0.31 to 3.5 +/- 0.42 ml/h (P < 0.05), and sodium excretion (UNaV) from 71.1 +/- 14 to 271.2 +/- 37 mueq/h (P < 0.001), there was a marked nocturnal inhibition of the response to LBPP, with no significant increases in V or UNaV. Urine osmolality decreased by more than 50% at both times of pressure exposure; potassium excretion was not significantly affected by either exposure and drinking was suppressed during daytine LBPP. Comparisons of the time course and diurnal regulation of the urinary responses suggest that several separate efferent control pathways are involved.