Cyclosporine administration in patients with organ transplants may cause cholestasis. In the rat, intraperitoneal administration of cyclosporine, 10 mg/kg, for three weeks did not cause liver function test abnormalities or hepatic histological lesions. However a significant reduction of bile flow and bile acid secretion rates was observed. The fact that reduction of bile flow was related to a decrease of the bile acid-independent flow suggests that cyclosporine-induced cholestasis results from an inhibition of bile acid secretion. Whether this inhibition is caused by the parental molecule or by cyclosporine metabolites needs to be clarified.