Fractional clearance of colloidal gold particles (k), liver weight and hepatic cytochrome P-450 were measured in rats with portacaval shunt and in rats with portacaval shunt plus arterialization of the hepatic stump of the portal vein. The effects of enzyme induction by phenobarbital was studied in both groups. Total arterialization of the liver provides a probably normal hepatic blood flow and seems to protect the liver from post-shunt atrophy. Nevertheless, in both arterialized and shunted rats, the cytochrome P-450 concentration was significantly lower than in controls. The same results were obtained after treatment by phenobarbital. These findings suggest that normal hepatic blood flow and oxygen supply are not the main determinant of a normal activity of the hepatic microsomal bio transformation system. Substances present in portal blood would probably be necessary in maintaining hepatic cytochrome P-450 level.