Dextromethorphan, a constituent of many over-the-counter cough syrups, is used as a probe drug for phenotyping subjects for their cytochrome P450 2D6 (CYP2D6) enzyme activity and for measuring CYP2D6 activity of preparations such as microsomes. In such studies, formation of the metabolite dextrorphan is used as indicator of the activity of this CYP enzyme. The present report describes an electron-capture gas chromatographic procedure developed for detection and quantification of dextrorphan in human liver microsomal preparations in vitro. After basification of the incubation mixture, dextrorphan was derivatized with pentafluorobenzoyl chloride under aqueous conditions prior to analysis on a gas chromatograph equipped with a capillary column, an electron capture detector, and a printer-integrator. Para-hydroxymephenytoin was carried through the procedure as internal standard. The procedure, which involves the derivatization of dextrorphan under aqueous conditions, is rapid and involves the use of the relatively economical procedure of electron-capture gas chromatography. The derivative is stable and possesses excellent chromatographic properties.