Short-term effects of cyclosporine were studied in the isolated perfused rat liver model. Bile flow was inhibited by cyclosporine in 2 mg/kg and 20 mg/kg doses but not by a 0.2 mg/kg dose. Cholestasis was accompanied by a decrease in bile acid secretion, indicating an inhibitory effect on the bile acid-dependent fraction of bile flow. Perfusate bilirubin levels increased threefold in rat livers given 20 mg/kg of cyclosporine, but did not change in control animals. Alkaline phosphatase and transaminase levels did not differ from those of control animals. The isolated perfused rat liver was able to excrete cyclosporine, as demonstrated by a continual decrease in perfusate cyclosporine levels. No light microscopic evidence of cholestasis or hepatocellular damage was demonstrated on histologic staining. Our model appears to be a good one for the study of altered hepatic physiologic characteristics caused by administration of cyclosporine.