The purpose of this study was to examine the effects of prostaglandin E1 (PGE1) on venous capacitance during controlled hypotension. Trinitroglycerin (TNG) was used as a control agent. In rats anesthetized with ketamine, mean arterial pressure was lowered to 70 mmHg and subsequently 50 mmHg by intravenous infusion of PGE1 or TNG. Venous capacitance was assessed before and during induced hypotension by measuring the mean circulatory filling pressure (MCFP). MCFP was measured after briefly arresting the circulation by inflating an indwelling balloon in the right atrium. MCFP was significantly decreased by PGE1 from 7.9 +/- 0.3 to 6.9 +/- 0.3 mmHg at mean arterial pressure of 70 mmHg and to 6.9 +/- 0.2 mmHg at mean arterial pressure of 50 mmHg. The decrease in MCFP by PGE1 at mean arterial pressure of 70 mmHg was not significantly different from TNG. However, the decrease in MCFP by PGE1 at mean arterial pressure of 50 mmHg was significantly less than that by TNG. The results suggest that the venous capacitance may be increased by PGE1 to a similar degree with TNG at doses to produce a comparable level of moderate hypotension, but the increase in venous capacitance may be less in PGE1 than TNG at doses to produce deep hypotension.