The in vitro metabolism of 1-3H-phenyl-1-cyclohexene (3H-PC) was studied in a crude microsomal preparation from mouse livers. The major routes of metabolism were allylic hydroxylation, oxidation of the allylic alcohol, and epoxidation-hydrolysis. The following metabolites were identified by comparison with reference compounds: 1-phenyl-1-cyclohexen-3-ol (major metabolite), 1-phenyl-1-cyclohexen-3-one (PC-3-one) (major), 1-phenyl-1-cyclohexen-6-ol (minor), 1-phenyl-1-cyclohexen-6-one (minor), and 1-phenylcyclohexane-1,2-diol (PC-1,2-diol) (minor). An additional metabolite, present in abundant quantities, was formed as a result of both allylic hydroxylation and epoxidation-hydrolysis. This triol contained hydroxyl groups at positions 1 and 2 of the cyclohexane ring but the position of the third hydroxyl group could not be established. PC-3-ol and PC-3-one were found to be somewhat more potent than PC in the inverted-screen test, whereas PC-1,2-diol was less effective. However, all three metabolites were considerably less active than PCP in this test.