These studies examined in vitro metabolism of phencyclidine (PCP) in a series of human liver microsomes (N = 10). Each sample was characterized for cytochrome P450 (CYP) content and for CYP1A, CYP2A, CYP2C, CYP2D, CYP2E, CYP3A, CYP4A, and lauric acid 11-hydroxylation metabolic activities. At least five PCP metabolites (c-PPC, t-PPC, PCHP, an unknown metabolite, and an irreversibly bound metabolite) were formed by the various human liver microsomes. Nevertheless, there was a large degree of inter-individual variation in the metabolite formation. For example, the irreversibly bound metabolite was formed in detectable amounts in only four of the ten samples. c-PPC, t-PPC and the irreversibly bound PCP metabolite formation rates significantly correlated with CYP3A activity. The CYP3A inhibitor troleandomycin was used to inhibit the formation of PCP metabolites. Troleandomycin inhibition was dose dependent with the highest dose producing complete inhibition of the formation of c-PPC, t-PPC, PCHP, and the irreversibly bound metabolite. In addition, PCP inhibited CYP3A-mediated testosterone 6 beta-hydroxylation by 50%. Furthermore, the relative intensity of CYP3A immunoreactive proteins significantly correlated with testosterone 6 beta-hydroxylation and with PCP metabolite formation (except for the unknown metabolite). PCHP formation also correlated with CYP1A activity, while the formation of the unknown PCP metabolite correlated with CYP2A activity. These studies suggest that several CYP isoforms contribute to PCP metabolism and that CYP3A plays a major role in PCP biotransformation in human liver microsomes.