Bleomycin inhibits hepatic microsomal ethylmorphine N-demethylase activity in both the athymic (nude) mouse and the rat four days after a single injection. This inhibition occurred both with subcutaneous and intraperitoneal injections. With multiple dose chronic treatment, s.c. injections resulted in decreases in both enzymatic activity and cytochrome P450 concentration in both species. However, i.p. chronic injection resulted in decreases only in the rat.