The effects of water diuresis, hypotonic NaCl, and hypotonic mannitol diuresis on renal sodium and water excretion were examined in normal dogs and in dogs with chronic constriction of the thoracic inferior vena cava and ascites (caval dogs). During all three diuretic states, the capacity to excrete solute-free water relative to the supply of sodium to the water clearing segment of the nephron was significantly greater in the caval dog. This finding was most evident during hypotonic NaCl diuresis but was also striking during hypotonic mannitol diuresis despite the more unfavorable gradient for sodium reabsorption at the distal tubule produced by this agent in caval dogs. In addition, fractional distal sodium load was significantly smaller in caval dogs during water diuresis and could not be increased as readily as in normal dogs by hypotonic NaCl or mannitol infusion. The data indicate that fractional sodium reabsorption is increased at the water clearing segment and the proximal tubule in caval dogs. The differences in the pattern of free water clearance and tubular sodium transport between normal and caval dogs could not be easily explained by alterations in renal hemodynamics or aldosterone secretion. It is suggested that in the caval dog an alteration occurs in other factors which might influence renal tubular sodium transport, such as intrarenal hemodynamics, renal interstitial volume or pressure, or a natriuretic hormone, leading to increased tubular sodium reabsorption.