Persistent secretion of vasopressin and/ or diminished distal fluid delivery have been proposed to explain the impaired water excretion associated with low-output cardiac failure. In the present investigation cardiac output (CO) was diminished in anesthetized dogs undergoing a water diuresis by constriction of the thoracic inferior vena cava (TIVC). In intact animals (group I) acute TIVC constriction decreased CO from 3.5 to 2.2 liters/min (P < 0.005) as urinary osmolality (U(osm)) increased from 103 to 543 mosmols/ kg (P < 0.001) and free water clearance (C(H2o)) decreased from 2.1 to -0.6 ml/min (P < 0.001). This antidiuretic effect was disassociated from changes in renal arterial and venous pressures, glomerular filtration rate, solute excretion, and renal innervation. To examine the role of vasopressin in this antidiuresis, studies (group II) were performed in acutely hypophysectomized, steroid-replaced animals. In these animals TIVC constriction decreased CO to a similar degree from 3.4 to 2.1 liters/min (P < 0.001). However, the effects on U(osm) (87-104 mosmols/kg) and C(H2o) (2.1-1.6 ml/min) were significantly less than in intact dogs. In another group of hypophysectomized animals, (group III) renal arterial and venous pressures were not controlled, and the effect of TIVC constriction on U(osm) was not significant (65-79 mosmols/kg) although C(H2o) decreased from 3.3 to 1.9 ml/min (P < 0.001). In both the group II and III studies, there were linear correlations between the changes in C(H2o) and the urine flow. Studies were also performed in baroreceptor-denervated animals with intact hypothalamo-neurohypophyseal tracts, and acute TIVC constriction altered neither U(osm) nor C(H2o) when renal arterial pressure was controlled. These results therefore indicate that the effect of TIVC constriction on U(osm) is primarily vasopressin mediated while the effect on C(H2o) is mediated both by vasopressin release and diminished distal fluid delivery. A decrease in renal arterial pressure, or some consequence thereof, seems to be an important determinant of the latter effect.