Head-up tilt (HUT) induces a reduction in preload, which is thought to be restored through sympathetic venoconstriction, reducing unstressed volume (V_u) and venous compliance (VeC). In this study, we assessed venous inflow and outflow responses, and their reproducibility, and determined the relation with autonomic function during HUT. Eight healthy non-pregnant women were subjected to 20° head-down to 60° head-up tilt (HUT), at 20° intervals. At each rotational step, we randomly assessed forearm volume-pressure curves (V-P curve; venous occlusion plethysmography) during venous inflow (VeC_IN) and outflow (venous emptying rate; VER_OUT). VeC_IN was defined as the ratio of the slope of the volume-time curve and the pressure-time curve, with direct intravenous pressure measurement. VER_OUT was determined using the derivate of a quadratic regression model, using cuff pressure. We defined V_u as the y-intercept of the V-P curve. We calculated for both methods the coefficients of reproducibility (CR) and variation (CV). Vascular sympathetic activity (LnLFsys) was determined by spectral analysis. VeC_IN decreased at each rotational step, as compared to supine position (p<0.05), whereas VER_OUT increased. CR of VeC_IN was higher in supine position than VER_OUT, but lower during HUT. CV varied between 19-25% (VeC_IN) and 12-21% (VER_OUT). HUT decreased V_u. The change in VeC_IN and VER_OUT correlated with the change in LnLFsys(r=-0.36, p<0.01 and r=0.48, p<0.01). This is the first study in which a reproducible reduction in VeC_IN and unstressed volume and a rise in VER_OUT during HUT are documented. The alterations in venous characteristics relate to changes in vascular sympathetic activity.