The ability of microsomes isolated from liver of pregnant ewes and their fetuses at near term to catalyze the biotransformation of benzo[a]pyrene, hexobarbital, meperidine, methadone and morphine was investigated. Cytochromes P-450 and b5, NADPH and NADH cytochrome c reductase, methadone and meperidine N-demethylase and morphine glucuronyltransferase activities were detected in microsomes from both maternal and fetal livers. Fetal hepatic microsomes however, lacked the ability to catalyze the hydroxylation of hexobarbital and benzo[a]pyrene.