A gas chromatographic mass spectrometric procedure using selected ion monitoring is described for the quantification of gallopamil in human plasma. Gas chromatographic separation of gallopamil from phenolic metabolite isomers is made possible by treatment with ethyl chloroformate. The detection limit for the quantitation by the present method is 0.09ng/ml of plasma. The method has sufficient sensitivity to permit pharmacokinetic studies with human subjects following the oral administration of gallopamil hydrochloride.