Young female Wistar rats were injected intraperitoneally with increasing doses of morphine (9-45 mg/kg b.w. twice a day) for 14 days. The activities of the cytochrome P-450-dependent monooxygenase system and microsomal electron transport chain I were determined. Long-term morphine treatment decreased significantly cytochrome P-450 content, while it did not affect microsomal electron transport chain I. A decrease in cytochrome b5 content was accompanied by a compensatory increase in the activity of NADH-cytochrome b5 reductase. Both 4-aminopyrine N-demethylase activity and aniline hydroxylase activity decreased after morphine treatment (48% and 70%, respectively, when compared with controls).