The response of plasma atrial natriuretic peptide (ANP) concentration to acute intravascular volume expansion was measured in ten male Wistar rats. An infusion of 3 ml polygelene colloidal solution at 37 degrees C over 45 s produced peak venous pressure rises of 1.5 cm water. A highly significant (P less than 0.001) rise of immunoreactive plasma ANP from 24.4 +/- 2.2 (mean +/- S.E.M.) pmol/l to a peak of 70.0 +/- 10.5 pmol/l occurred within 2.5 min. Plasma ANP concentrations had virtually returned to basal levels (32.7 +/- 2.7 pmol/l) 30 min after this acute volume load. A further infusion of 10 ml polygelene colloidal solution in 2 min produced peak venous pressure rises of 10 cm water and caused a dramatic and significant (P less than 0.001) increase of plasma ANP concentration to a peak of 534.8 +/- 38.5 pmol/l, occurring 7.5 min after infusion. The plasma ANP concentration had fallen but remained above basal levels 30 min later (137.2 +/- 26.4 pmol/l). Similar results were obtained using an identical protocol but with whole rat blood instead of polygelene solution as the volume-expanding agent. Gel column chromatography suggested that the majority of the immunoreactive ANP in rat plasma was of similar molecular size to rat alpha-ANP (1-28). These results support the hypothesis that blood volume expansion is a potent stimulus for the release of ANP into plasma.