Amrinone, a new bipyridine derivative, exerts a positive inotropic action in experimental preparations and is effective when administered orally to dogs. To assess its immediate effects in man, we studied by cardiac catheterization the hemodynamic responses to amrinone (1.85 to 3.5 mg per kilogram given intravenously) in eight patients with congestive heart failure already receiving full doses of digitalis. the following statistically significant (P less than 0.01) effects were noted: cardiac index increased from a mean +/- 1 S.D. of 1.8 +/- 0.3 to 2.6 +/- 0.3 liters per minute per square meter; peak rate of left ventricular pressure rise rose from 849 +/- 233 to 1206 +/- 456 mm Hg per second; left ventricular end-diastolic pressure fell from 25 +/- 9 to 14 +/- 7 mm Hg; pulmonary-capillary pressure fell from 28 +/- 8 to 15 +/- 4 mm Hg; and right atrial pressure fell from 12 +/- 6 to 7 +/- 5 mm Hg. Mean heart rate was unchanged, and aortic mean pressure declined slightly (86 +/- 10 to 80 +/- 7 mm Hg, P less than 0.025). No toxicity was observed. Amrinone, whose mechanism of action has not yet ben defined, warrants further study as a possible treatment for heart failure.