Infusion of atriopeptin into humans and animals induces diuresis, natriuresis, hemodynamic changes, and an increase in arterial hematocrit. The objective of the present study was to elucidate the mechanism(s) responsible for the increase in hematocrit in rats given atriopeptin-24 (AP-24). Infusion of AP-24 for 30 minutes increased large vessel and total vascular hematocrits by 10-15percent; while decreasing microvascular hematocrits by 9-26percent; in numerous tissues. Regional vascular permeation by [131I] bovine serum albumin was markedly increased (2-5.6-fold) in many tissues, consistent with a 16percent; decrease in plasma volume. AP-24 infusion had no effect on extracellular fluid volume or the volume of circulating red cells. Vascular resistance was decreased and was associated with a significant increase in blood flow in many, but not all, tissues. In the atrium and in the small and large intestine the percentage decrease in microvascular hematocrit exceeded the increase in blood flow. These observations indicate that the increase in large vessel hematocrit induced by AP-24 infusion 1) is accompanied by a decrease in (microvascular) hematocrit in many tissues, 2) reflects an increase in overall (i.e., total vascular hematocrit), and 3) is the consequence of a decrease in plasma volume resulting from a marked increase in the rate of vascular permeation by plasma constituents in multiple tissues.