Trout are of interest in defining the relationship between fluid and salt balance on cardiovascular function because they thrive in freshwater (FW; volume loading, salt depleting), saltwater (SW; volume depleting, salt loading), and FW while fed a high-salt diet (FW-HS; volume and salt loading). The effects of chronic (>2 wk) adaptation to these three protocols on blood volume (51Cr red cell space), extracellular fluid volume (99mTc-diethylene triaminepenta-acetic acid space), arterial (dorsal aortic; P(DA)) and venous (ductus Cuvier; Pven) blood pressure, mean circulatory filling pressure (zero-flow Pven), and vascular capacitance were examined in the present study on unanesthetized rainbow trout. Blood volume, extracellular fluid volume, P(DA), Pven, and mean circulatory filling pressure progressively increased in the order SW < FW < FW-HS. Vascular capacitance in SW fish appeared to be continuous with the capacitance curve of FW fish and reflect a passive volume-dependent unloading of the venous system of FW fish. Vascular capacitance curves for FW-HS fish were displaced upward and parallel to those of FW fish, indicative of an active increase in unstressed blood volume without any change in vascular compliance. These studies are the first in any vertebrate to measure the relationship between fluid compartments and cardiovascular function during independent manipulation of volume and salt balance, and they show that volume, but not salt, balance is the primary determinant of blood pressure in trout. They also present a new paradigm with which to investigate the relative contributions of water and salt balance in cardiovascular homeostasis.