Dimemorfan (d-3-methyl-N-methylmorphinan), an analogue of dextromethorphan, is commonly used as a non-opioid antitussive. To clarify the contribution of cytochrome P450 (P450) in dimemorfan N-demethylation, effects of selective inducers and inhibitors were studied in ICR mice. Phenobarbital (PB)- and dexamethasone (Dex)-treatments caused 5-fold increases of liver microsomal dimemorfan N-demethylation activity. In untreated mouse liver microsomes, demethylation activity was strongly inhibited by a CYP3A inhibitor, ketoconazole. In PB-and Dex-treated mouse liver microsomes, ketoconazole caused strong inhibition, whereas orphenadrine caused a decrease of less than 20%. Pretreatment of control mouse liver microsomes with anti-CYP3A inhibited demethylation activity, whereas pre-treatment with anti-CYP2B had no effect. In PB-and Dex-treated mouse liver microsomes, the demethylation activity was inhibited by both anti-CYP3A and anti-CYP2B. In control mice, the intrinsic clearance of dimemorfan from N-demethylation was 5.8 microl min(-1)mg protein(-1). In PB- and Dex-treated mice, the correlation coefficient of fitting using one-enzyme and two-enzyme models were similar. The intrinsic clearances of induced mouse liver microsomes were similar. These results revealed that CYP3A played a major role in hepatic demethylation in untreated mice. Both CYP3A and CYP2B were involved in this demethylation in PB- and Dex-treated mice.