Studies on the metabolic bioactivation of the psychotomimetic amine phencyclidine have been pursued through the characterization of a new metabolite which is formed via initial cytochrome P-450 catalyzed oxidation of the parent drug to the corresponding iminium species. CI mass spectrometric and diode array UV and 1H NMR spectral analyses provided evidence for the conjugated amino enone compound, 1-(1-phenylcyclohexyl)-2,3-dihydro-4-pyridone. Confirmation of the proposed structure was achieved by comparing the 1H NMR and high-resolution EI mass spectral properties of the metabolic isolate with the corresponding spectra of an authentic synthetic sample. Possible intermediates involved in the formation of the dihydropyridone metabolite from the phencyclidine iminium ion are discussed in terms of structural analogies to reactive intermediates formed in the bioactivation of the nigrostriatal toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).