Effects of recombinant bovine somatotrophin (r bst) on in vivo and in vitro oxidative drug metabolism were studied in male rats. r bst was given subcutaneously at a dose of 250 or 500 μg 100 g −1 bodyweight 24 h −1 in different dosage patterns. Sulphadimidine ( sdd) plasma clearance, urinary excretion of 6-hydroxy- sdd and the in vitro microsomal sdd-hydroxylations were only inhibited when r bst was given in three injections per 24 hours. The hepatic microsomal ethylmorphine N-demethylation rate and the testosterone hydroxylation rate at the 6β position were significantly reduced after one r bst injection per 24 hours. Microsomal testosterone hydroxylation rates at the 16α and 2α-positions were reduced depending on the frequency of r bst administration. It is concluded that the inhibition of in vivo and in vitro drug oxidation in rats by r bst is associated with selective changes in activity of cytochrome P450 enzymes in the liver.