To investigate the acute effect of sustained volume expansion in patients with chronic hepatic ascites, renal and hemodynamic studies were performed on six patients receiving the peritoneovenous shunt. Within 1 hr of shunt insertion, the cardiac output rose by 20% to 60% above the preoperative level and the renal blood flow by 70% to 300%. At the same time both the plasma renin activity and serum aldosterone levels fell to a mean of 34% of the preoperative levels. The urine output increased from fourfold to 38-fold, but the sodium excretion increased in only two of the six patients. The data show that the acute effect of sustained volume expansion is predominantly on water rather than on sodium excretion. This manoeuver acutely suppressed circulating renin and aldosterone levels; however, the blunted natriuretic response suggests that other factors are involved in the sodium retention in these patients.