A sensitive (to 0.5 ng/ml) and specific method for the determination of verapamil and norverapamil which utilizes gas-liquid chromatography with nitrogen-phosphorus detection is described. A basic extraction with acid back-wash and final basic reextraction is used for the preparation of plasma samples. Standard curves using alpha-isopropyl-alpha-[(N-methyl-N-homoveratryl)-beta-aminoethyl]- 3,4- dimethoxyphenylacetonitrile hydrochloride (D-517) are linear for concentrations from 0.5 to 200 ng/ml for both verapamil and norverapamil. Within-day and between-day reproducibility is good with a coefficient of variation less than 10% for all concentrations. Recovery is complete for both verapamil and norverapamil. Application of the method is demonstrated by a pharmacokinetic study in a normal volunteer who received 10 mg verapamil hydrochloride by intravenous infusion.