A GLC assay for phencyclidine (PCP) is described, which also simultaneously measures three primary hydroxylated metabolites formed from incubating PCP in tissue homogenates. Using the FID detector, the limits of reliable detection of PCP and both monohydroxy metabolites, 4-phenyl-4-piperidino-cyclohexanol, 2, and 1-(1-phenylcyclohexyl)-4-hydroxypiperidine, 3, are 0.02 mumol per injection and 0.05 mumol for the dihydroxy metabolite, 4-(4'-hydroxypiperidino)-4-phenylcyclohexanol, 2A. Baseline separation of an compounds was achieved and coefficients of variation (between-run) was 3-6% for PCP, and both monohydroxy metabolites, and 12% for the dihydroxy metabolite. A GCMS assay is also reported herein for the analysis of PCP at low levels, and can detect 5 pmol per injection of PCP, with a linear standard curve from 50 to 2000 pmol.