The oral effectiveness of 10 mg followed by 20 mg of isosorbide dinitrate in 21 patients with acute mycardial infarction was studied over a period of 13 hours. The patients were grouped according to initial left ventricular filling pressure: group I, pressure less than 20 mm Hg, and group II, pressure more than 20 mm Hg. Patients in group II had left ventricular failure. In both groups isosorbide dinitrate resulted in a significant decrease in pulmonary arterial pressure. The left ventricular filling pressure decreased in group I from 13.6 +/- 4.0 to 7.1 +/- 2.6 mm Hg (mean +/- 1 standard deviation) and in group II from 26.9 +/- 4.6 to 19.0 +/- 3.6 mm Hg (P less than 0.001). Cardiac output decreased in group I from 5.1 +/- 1.0 to 4.5 +/- 0.9 liters/min, whereas in group II it increased significantly from 3.5 +/- 0.8 to 4.1 to 0.9 liters/min (P less than 0.001). In both groups, peripheral arterial blood pressure decreased (P less than 0.60). Heart rate remained constant. Whether cardiac output increased or decreased was found to be dependent on the initial left ventricular filling pressure. In patients with an initially high value (above 20 mm Hg), the increase in cardiac output is probably due to the reduction of afterload. An additional factor may be the decrease in left ventricular filling pressure, which leads to an improved blood supply in the affected mural segments as a result of the decrease in the extravascular component of the coronary resistance. Significant changes in cardiac output and left ventricular filling pressure were achieved 3 to 5 hours after oral administration of isosorbide dinitrate. Clinical signs of failure were less pronounced. Isosorbide dinitrate is, therefore, a therapeutic agent in the treatment of left ventricular failure due to acute myocardial infarction.