The onset of sodium retention in the phenobarbital and carbon tetrachloride model of cirrhosis in the rat is preceded by a linear decrease in hepatic function as measured by the aminopyrine breath teat. Sodium retention occurs when liver function decreases below a critical threshold. Changes in systemic hemodynamics may be responsible for initiating the development of renal sodium retention. The objective of this study was to investigate the relationship between hepatic function and systemic and renal hemodynamics of experimental cirrhosis in rats maintained on a constant salt diet. Cirrhosis was induced in phenobarbitaltreated rats by weekly administration of carbon tetrachloride. The aminopyrine breath test served as a measure of hepatic function. Three groups of animals were studied to evaluate the contribution of changes in systemic and renal hemodynamics to the onset of sodium retention: a group with sodium retention and aminopyrine breath test results just below the critical threshold, a group without sodium retention and aminopyrine breath test results just above the critical threshold and a phenobarbital-treated control group. In each group, urinary sodium excretion, renal plasma flow, glomerular filtration rate, mean arterial pressure and arterial and renal venous plasma renin activities were determined. A progressive, significant reduction in mean arterial pressure was seen, comparing controls with the other two groups. No differences in renal plasma flow were observed between the three groups, but glomerular filtration rate and filtration fraction were slightly reduced in the sodium-retaining group comparedwith the non-retaining group and controls. A trend toward increased renin activity was observed in both groups of cirrhotic animals, but the differences were not statistically significant, compared with controls. A positive, curvilinear relationship was found between aminopyrine breath test results and mean arterial pressure and glomerular filtration rates, and a negative association was found between aminopyrine