To investigate the role of nitric oxide in renal function and hemodynamics in cirrhotic patients with ascites, L-arginine (30 g in 300 mL of distilled water), a substrate for nitric oxide synthase, was infused into six cirrhotic patients with ascites, and the effects were compared with those of saline infusion. Healthy controls (n = 5) were also studied under the same conditions. In the patients, L-arginine infusion significantly decreased systolic and diastolic blood pressures while markedly increasing urinary flow and urinary sodium excretion; no significant changes were seen with saline infusion. In controls, only diastolic blood pressure was decreased by L-arginine infusion, whereas urinary flow and urinary sodium excretion were increased by both L-arginine and saline infusion. In both groups, a similar increase of plasma atrial natriuretic factor (ANF) was seen with L-arginine and saline infusions; endotheline and cate-cholamines were not affected by either infusion. In both groups, plasma levels of vasopressin were increased by L-arginine infusion. In the cirrhotic patients, urinary excretions of cyclic guanosine monophosphate (cGMP) and nitrates/nitrites (NOx) were significantly increased by L-arginine infusion, whereas no significant changes were seen with saline infusion. In controls, only the excretion of cGMP was increased by L-arginine infusion. In summary, L-arginine infusion induces diuresis and natriuresis accompanied by increased excretions of cGMP and NOx in cirrhotic patients with ascites. This differs from the response in controls, where the increase in urinary sodium excretion is not accompanied by an increase in markers of increased nitric oxide synthesis. (Hepatology 1995; 22:1430–1435).