Mean circulatory filling pressure (MCFP) has been measured after vagally induced cardiac arrest in 11 nonpregnant and 10 near-term pregnant rabbits, anesthetized with pentobarbital sodium. MCFP was 6.1 +/- 0.4 (SD) mmHg in the nonpregnant and 7.2 +/- 0.4 mmHg in the pregnant animals. The difference of 1.1 mmHg was significant (P < 0.001). Sympathetic blockade with bretylium tosylate (10 mg/kg) reduced MCFP by 0.4 mmHg in both nonpregnant and pregnant rabbits. Vascular capacitance was examined in the two groups. Unstressed vascular volume and vascular compliance were derived from measurements of MCFP after increasing blood volume by 8 and 16% or reducing it by 8%. The unstressed vascular volumes, 33.9 +/- 3.9 (SD) ml/kg in the nonpregnant and 35.1 +/- 3.2 ml/kg in the pregnant group, were not significantly different, but compliance in the pregnant group (4.0 +/- 0.6 ml.kg-1.mmHg-1) was significantly greater than in the nonpregnant rabbits (3.4 +/- 0.6 ml.kg-1.mmHg-1) (P < 0.05). We conclude that there are changes in vascular capacitance in rabbit pregnancy, probably not related to alterations in vasomotor activity, but these are insufficient to fully compensate for the increase in blood volume, thus leading to the rise in MCFP.