A sensitive gas-chromatographic method for quantitative analysis of verapamil in human plasma is described. The method involves a single extraction procedure, followed by separation on a capillary column and detection with a nitrogen-phosphorus detector. The detection limit, based upon an assayed plasma volume of 0.5 ml, is 2 ng/ml. The standard curve is linear in the concentration range of 2 to 1000 ng/ml. The recovery of verapamil by pentane-isopropanol extraction was found to be 95%. Zipeprol is used as the internal standard. No interference from drugs needed for the associated cancer therapy has been found. Serum verapamil concentrations are determined by this method in fourteen cancer patients undergoing treatment with adriamycin.