The oral effectiveness of isosorbide dinitrate (Isoket) was tested in 21 pateints with acute myocardial infarction. Patients were divided into two groups according to thier initial left-ventricular filling pressure (more or less than 20 mm Hg). In both groups there was a highly significant decrease in pulmonary artery pressure. In group I (less than 20 mm Hg) the left-ventricular filling pressure fell from 13.6 +/- 4.0 to 7.1 +/- 2.6 mm Hg, while in group II it fell from 26.9 +/- 4.6 to 19.0 +/- 3.6 mm Hg (P less than 0.001). The cardiac output fell in group I from 5.1 +/- 1.0 to 4.5 +/- 0.9 l/min, while in group II there was a significant rise from a low value (3.5 +/- 0.8 l/min) to 4.1 +/- 0.9 l/min (P less than 0.001). In both groups there was a slight but not significant fall in arterial blood pressure, while heart rate remained constant.