Plasma vasopressin (AVP) levels are often elevated in congestive heart failure (CHF). To determine the significance of AVP in CHF, we performed clearance studies on the UM-X7.1 strain of cardiomyopathic (CM) hamsters with moderate heart failure and age-matched healthy controls. Exogenous AVP or a selective V<sub>2</sub> agonist (0.3 ng·kg<sup>–1</sup>·min<sup>–1</sup>) reduced the fractional excretion of sodium (FE<sub>Na</sub>) and water (FE<sub>H2</sub>O) by 40–46% in the control group. Although the CM hamsters exhibited a blunted physiological response to the V<sub>2</sub> agonist, their urinary cAMP levels were fivefold that of normal and reflect an altered regulation of V<sub>2</sub> receptor signalling during CHF. Additional studies also showed that infusion of a V<sub>2</sub> antagonist (0.3 ng·kg<sup>–1</sup>·min<sup>–1</sup>) produced natriuresis and diuresis in CM hamsters (FE<sub>Na</sub>: 7.9 ± 1.1 vs. 4.8 ± 0.6%, p < 0.05; FE<sub>H2</sub>O: 2.2 ± 3 vs. 1.5 ± 0.2%, p < 0.05) but did not decrease fluid reabsorption in the n
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