A simple high-performance liquid chromatographic method for the simultaneous measurement of plasma verapamil and norverapamil concentrations has been developed. The sample (100 microliters) is vortex-mixed for 30 sec with 4 M sodium hydroxide solution, pH 13 (50 microliters), internal standard solution (aqueous 5,6-benzoquinoline, 0.20 mg/l) (50 microliters) and methyl tert.-butyl ether (200 microliters). After centrifugation at 9950 x g for 2 min, a portion (100 microliters) of the resulting extract is analysed on a microparticulate (5 microns) silica column using a methanolic solution of potassium bromide (3.0 mM) and perchloric acid (0.37 mM) as the mobile phase, and the column effluent is monitored by fluorescence detection using an excitation wavelength of 203 nm. A specimen, together with a quality control sample, can be analysed, in duplicate, within 30 min. The limit of accurate measurement of the assay is 2 micrograms/l, and no potential sources of interference have been identified. The method has advantages of speed, small sample requirement and complete resolution of the three major metabolites of verapamil.