Vascular capacitance was studied in anesthetized control (CONT) animals and in rats after electrolytic ablation of the periventricular tissue surrounding the anteroventral third cerebral ventricle (AV3V-X). Blood volume (BV) was determined by use of radiolabeled serum albumin, and mean arterial pressure (MAP) and central venous pressure (CVP) were continuously measured. Mean circulatory filling pressure (MCFP) was calculated by use of MAP and CVP obtained during circulatory arrest induced by inflation of a right atrial balloon during BV expansion and contraction. MCFP-BV relationships were calculated to estimate vascular compliance. CONT and AV3V-X animals were tested after treatment with both vehicle and hexamethonium, a ganglionic blocking agent. BV, MAP, and CVP were similar between CONT and AV3V-X animals. However, MCFP was significantly lower in AV3V-X animals (4.6 +/- 0.3 mmHg) than in CONT rats (6.6 +/- 0.5 mmHg). Furthermore, AV3V ablation caused a significant shift of the MCFP-BV relationship toward the volume axis with no change in compliance, indicating decreased venous tone. Finally, hexamethonium treatment significantly reduced MCFP in CONT animals (3.8 +/- 0.7 mmHg) and shifted the MCFP-BV line toward the BV axis but had no effect on these measures in AV3V-X animals. These data indicate that electrolytic ablation of AV3V periventricular tissue significantly reduces venous tone by decreasing neurally mediated venoconstriction.