Vascular capacitance was studied in conscious early-phase (less than 6 wk) 2-kidney, 1-clip (2K, 1C) hypertension and compared with sham-clipped control rats. Two other groups of 2K, 1C rats were studied before and 6 h after unclipping or a sham operation. Mean circulatory filling pressure (MCFP) was measured during a brief circulatory arrest caused by inflation of a right atrial balloon. Blood volume (BV) was determined from plasma volume (125I-labeled albumin) and hematocrit. MCFP was measured at resting BV and after rapid BV changes. Vascular compliance was derived from the MCFP-BV curve. Hypertensive 2K, 1C rats had an increase in hematocrit (46 +/- 1.3 vs. 42 +/- 0.4%, P less than 0.01) and no difference in BV compared with controls. MCFP was increased (8.6 +/- 1.0 vs. 7.2 +/- 0.2 mmHg, P less than 0.01) with no difference in compliance, indicating decreased unstressed vascular volume in the 2K, 1C group. After unclipping, there was a significant fall in mean arterial pressure to normal, with a fall in MCFP (8.14 +/- 0.32 to 6.78 +/- 0.11 mmHg, P less than 0.01), but there was no difference in BV or compliance compared with the 2K, 1C group, indicating an increase in unstressed vascular volume after unclipping. These studies for the first time show an important role for vascular capacitance in modulating the circulatory changes accompanying the fall in blood pressure in surgical reversal of 2K, 1C hypertension.