Moderate physical exercise does not affect glomerular filtration rate (GFR) and renal excretory function in normal subjects. This study is aimed at assessing the effects of moderate physical exercise on renal function in 21 nonazotemic cirrhotic patients with ascites. Arterial pressure, heart rate, and renal function were assessed in the patients after 2 hours in the supine position and during 30 minutes of moderate cycloergometric exercise in the sitting position. The activity of the renin-aldosterone and sympathetic nervous systems and the plasma levels of antidiuretic hormone (ADH) and atrial natriuretic peptide (ANP) were determined at the end of each period. Physical exercise induced a marked reduction in GFR (75 ± 10 to 49 ± 6 mL/min), free-water clearance (6.1 ± 1 to 3.4 ± 1 mL/min), and sodium excretion (7.8 ± 2 to 4.3 ± 1 μEq/min) in 10 patients (Group I). In the remaining 11 cases (Group II) there were no changes in these parameters. Renal perfusion significantly decreased in both groups although the reduction was greater in Group I (-34.7% ± 4.6% vs. -7.5% ± 3.1%, P < .001). Physical exercise was associated with a significant and comparable increase in arterial pressure, heart rate, and plasma levels of renin, aldosterone, and norepinephrine (NE) in the two groups of patients. The ANP concentration did not change. Patients from Groups I and II differed significantly (P < .05) only in plasma renin activity (PRA) and NE concentration, which were higher in Group I patients both in the supine rest (renin: 4.7 ± 1.6 vs. 1.4 ± 0.5 ng/mL x h; NE: 576 ± 115 vs. 288 ± 42 pg/ mL) and during exercise (renin: 7.1 ± 1.8 vs. 2.6 ± 1 ng/ mL x h; NE: 925 ± 135 vs. 630 ± 90 pg/mL). In conclusion, moderate physical exercise has no detrimental effects on renal function in cirrhotic patients with ascites with no or mild activation of the renin-aldosterone and sympathetic nervous systems. However, moderate physical exercise causes a marked impairment in the renal function of patients with ascites with marked stimulation of these vasoconstrictor systems.