The transformation of phencyclidine in a mouse liver microsome preparation to several oxidative metabolites was studied. With use of GLC and HPLC methods with internal standards, phencyclidine and six metabolites were quantitated and the amino acid 12, resulting from the alpha-oxidation of the piperidine ring, was produced in 10-50 times greater amounts than the other metabolites. While most piperidines and pyrrolidines produce an amino acid and a corresponding lactam, it was found that phencyclidine was not converted to the lactam 11.