A quantitative gas chromatographic assay for the determination of phenelzine in plasma or other fluids is presented. A stable derivative is formed by cyclization with acetylacetone, and the derivative is separated, purified, and concentrated by liquid-liquid solvent extraction. Analysis of the extracts is by capillary gas chromatography with nitrogen specific detection. Detection limits of less than 1 ng/mL and CVs of 5% at the 10-ng/mL level are attainable with a 2-mL sample and benzylhydrazine as the internal standard.