The metabolism of [3H]-3-methylcholanthrene (3-MC) was studied in the isolated, perfused rat liver. Following addition of 250 mug to the perfusion fluid, 3-MC disappeared rapidly. After 2 hr, approximately 34% of the radioactivity was excreted in the bile, 6% remained in the perfusate, and 60% was found in the liver. Of the liver radioactivity, 80% was unchanged 3-MC, 11% was conjugated metabolites, 4% was free hydroxymetabolites, and 4% was nonextractable, presumably bound to macromolecules. Of the perfusate radioactivity, 82% was conjugated metabolites, 3% was free hydroxymetabolites, and 15% was unchanged 3-MG. A similar distribution was observed in intact, bile-cannulated rats, but biliary excretion was about one-fourth as high with double the i.v.-injected dose. Biliary excretion in perfused livers rose rapidly during the first 30 to 40 min, then decreased steadily. It was nearly twice as high in male as in female rat livers. Pretreatment of rats with 3-MC more than doubled the biliary excretion rate over the first 20 to 30 min in livers of both sexes and raised that of the female to that of the male rat liver. Neither retinol acetate nor 7,8-benzoflavone had any appreciable effect on biliary excretion of 3-MC metabolites. 2-Diethylaminoethyl-2,2-diphenylvalerate, a well-known microsomal oxygenase inhibitor, lowered excretion by 80 to 90% and lengthened the lag period, and dibutyryl-3',5'-cyclic adenosine monophosphate markedly increased the rate of excretion of 3-MC metabolites. Fractionation of bile by chromatography on Sephadex LH-20 revealed six well-defined peaks of radioactivity. In contrast, bile of intact rats given 3-MC gave a pattern on Sephadex chromatography consisting of only three peaks. Preliminary data suggest that these consist of conjugates of dihydroxymetabolites as well as more highly hydroxylate derivatives. The data obtained indicate that the perfused liver is an appropriate experimental model for studies on the hepatobiliary metabolism of carcinogens.