This article describes a selective gas chromatographic method for the resolution and quantification of phenoperidine and its two metabolites, pethidine (meperidine) and norpethidine (normeperidine). Drugs and SKF 525 A, the internal standard, are separated from plasma by solvent extraction under alkaline conditions. They are chromatographed on a 3% OV-17 Chromosorb Q glass column and detected with a nitrogen-phosphorous detector. Linearity is observed in the study range (5-200 ng/ml). No interference by endogenous substances is noted.