The short-term hemodynamic response to atrial natriuretic peptide appears to be partly mediated by decreased venous return, which could result from increased circulatory capacitance or decreased blood volume. To determine if rat atrial natriuretic peptide 99-126 (0.5 microgram/kg/min IV for 30-70 minutes) dilated capacitance vessels or decreased blood volume, mean circulatory filling pressure (measured during brief circulatory arrest by inflating an intraatrial balloon) and blood volume (51Cr-erythrocytes) were measured in anesthetized rats. Mean circulatory filling pressure, central venous pressure, and blood volume decreased by 0.4 mm Hg, 0.5 mm Hg, and 3.4 ml/kg, respectively. To determine the total circulatory pressure-volume relationship without influence from autonomic reflexes, mean circulatory filling pressure and blood volume were measured in spinal-cord-transected rats before and immediately after infusing or withdrawing 5 ml blood. Atrial natriuretic peptide decreased mean circulatory filling pressure, central venous pressure, and blood volume by 0.9 mm Hg, 1.7 mm Hg, and 8.0 ml/kg, respectively, and displaced the pressure-volume relationship toward the pressure axis by decreasing extrapolated unstressed volume. Similar results were obtained in spinal-cord-transected rats that had initial vascular tone restored to a greater level by norepinephrine infusion. In anephric rats, atrial natriuretic peptide decreased central venous pressure by 0.3 mm Hg and blood volume by 1.6 ml/kg. The results indicate that short-term infusion of atrial natriuretic peptide reduced circulatory capacitance in rats and suggest that this reduction resulted from diminished blood volume due to urinary fluid loss followed by passive vascular recoil and active venoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)