An isocratic high-performance liquid chromatography method for the assay of plasma amrinone is described. Plasma amrinone is extracted using protein precipitation with an internal standard, separated with a reverse phase column, and detected using ultraviolet absorption. Each run is completed within 10 min. The assay can detect amrinone concentrations between 0.5 and 10.0 micrograms/ml, within the accepted therapeutic range. The assay has a within-day coefficient of variation of less than 5% and a day-to-day coefficient of variation of less than 10% in the therapeutic range of amrinone. This technique is an accurate, simple, and rapid method for the determination of amrinone concentrations in plasma.