On one day six male subjects underwent an upright seated (SEAT) study, and on another day they were subjected to a head-down tilt of 3 degrees (HDT). Compared with SEAT, HDT induced prompt increases in central venous pressure (CVP) from -0.5 +/- 0.8 to 8.3 +/- 0.3 mmHg (P < 0.001) and in arterial pulse pressure of 8-18 mmHg (P < 0.001). CVP stabilized after 6 h at levels 2.4-2.8 mmHg below the peak value. Simultaneously, renal sodium excretion gradually increased over the initial 5 h of HDT and stabilized at a level approximately 125 mumol/min over that of SEAT (P < 0.001). Urine flow rate and solute free water clearance increased during the initial 2-6 h of HDT (P < 0.001) but returned to the level of SEAT thereafter. We concluded that CVP is slightly reduced over 12 h of HDT and that a clear temporal dissociation exists between renal sodium and water handling. We suggest that the combined effect of the sustained suppressions of plasma renin activity and plasma aldosterone and norepinephrine concentrations constitutes a mechanism of the increase in renal sodium excretion.