A recent marked increase in “street-drug” phencyclidine use and the subsequently associated overdoses has made development of an assay for the drug in biological fluids important. Phencyclidine in biologic samples was extracted into petroleum ether which was dried, and the residue obtained on drying was redissolved in chloroform for gas chromatographic analysis with a flame ionization detector. Ketamine was used as an internal standard. The method was used to determine the diagnosis of phencyclidine overdose in nine patients and to follow the gradual improvement in clinical course. The plasma half-life of phencyclidine is approximately 11 hr.