Dextromethorphan undergoes O-demethylation to dextrorphan and N-demethylation to 3-methoxymorphinan. 3-Hydroxymorphinan, a didemethylated compound, is secondarily formed. The O-demethylation pathway to dextrorphan is polymorphic and under CYP2D6 genetic control. Adult and fetal studies were performed to characterize the cytochrome P450 families involved in dextromethorphan metabolism and their ontogeny. In adult volunteers in vivo and in vitro studies demonstrate that the O-demethylation pathway to dextrorphan is dependent on CYP2D6 and is predominant in extensive metabolizers and defective in poor metabolizers of the drug. The N-demethylation pathway to 3-methoxymorphinan is accessory and is dependent on the CYP3A subfamily. In human fetal microsomes, CYP2D6 protein and activity are not detectable until birth, while CYP2D6 RNA is present in significant amounts before birth. CYP3A activity is detectable in large amounts as early as the 17th week of gestation. Fetal and adult members of the CYP3A subfamily have close, although different, properties, as demonstrated by immunoinhibition studies.