1. It has been suggested that endogenous renal prostaglandin E (PGE) constitutes a determinant of renal haemodynamics and renal sodium handling in patients with cirrhosis. We have accordingly assessed the effects of augmenting endogenous prostaglandins on renal function. We utilized water immersion to the neck since previous studies demonstrated that the redistribution of blood volume and concomitant central hypervolaemia thus induced produces a prompt and marked augmentation of PGE excretion in normal man. 2. Thirteen cirrhotic patients were studied twice while in balance on a daily 10 mmol of sodium/100 mmol of potassium diet during control and during water immersion. Urinary PGE was determined hourly for 6 h. 3. Cirrhotic patients manifested a wide continuum of responses characterized by either a sluggish or barely discernible natriuretic response (n = 5) or an appropriate natriuretic response (n = 8). 4. Water immersion to the neck resulted in a highly significant increase in mean UPGEV, which was threefold that manifested by normal subjects studied under identical conditions. Furthermore, cumulative sodium excretion during immersion correlated with PGE excretion (P less than 0.05). 5. These findings, together with the results of studies utilizing prostaglandin synthase inhibitors, are consistent with the postulate that renal PGE may play a role in the alterations of renal function in decompensated cirrhosis.