High-performance liquid chromatographic methods for the analysis of amrinone in plasma and for both amrinone and its N-acetyl metabolite in urine were developed and applied to measure specimens obtained from a number of healthy men who had received intravenous or oral amrinone. The intravenous doses ranged from 0.8 to 2.2 mg/kg. Terminal elimination of amrinone from the bloodstream followed apparent first-order kinetics. Half-life, after the drug had distributed to the tissues, was estimated by a log-linear least-squares regression; mean half-life was 2.6 +/- 1.4 hr. During the first 24 hr after medication, unchanged amrinone excreted in the urine of these subjects represented 10% to 40% of the dose. N-Acetyl metabolite in the urine represented less than 2% of the dose. In the oral study, doses ranged from 25 to 250 mg (0.31 to 3.5 mg/kg) and the maximum plasma concentration attained was proportional to the dose. The first order terminal elimination half-life was possibly dose-related. In only one subject were there unequivocal amounts of the N-acetyl metabolite in the plasma.