Abstract: A sensitive high-performance liquid chromatographic assay suitable for thesimultaneous determination of verapamil(Ⅰ)and its major active metabolite norverapamil(Ⅱ)inhuman plasma is described. After adding internal standard ethmosine, plasma samples were extractedusing a mixture of n-hexane and n-butyl alcohol(12 ∶ 1)to give mean recoveries of>92%of bothⅠ, and Ⅱ. The extracts were chromatographed on a C₁₈ reversed phase column with a mobile phasecompcosed of methanol,water and triethylamine(67 ∶ 33 ∶0.4,pH 6.7), with UV(λ,279 nm)detection. The calibration curves were linear over a wide range of concentrations(25～1000ng·ml^-1), and the limits of determination was 2.5 ng·ml^-1 for Ⅰ and 5.0 ng·ml-1 for Ⅱ. Themethod showed good precision: the within-day RSD were <7.6%for both Ⅰ and Ⅱ; the day-to～dayRSD were <8.6%for beth Ⅰ and Ⅱ.Using this assay, plasma concentrdtions of both Ⅰ and Ⅱ weresimultaneously determined in six volunteers after a single oral dose of 120 mg of verapamil· HCl.