We performed continuous electrocardiographic ST-segment monitoring and serial coronary arteriography in 45 consecutive patients presenting in the early stages of acute myocardial infarction. During cardiac catheterization, 28 episodes of arteriographically confirmed coronary reopening and subsequent reocclusion were observed in 16 patients before (3 episodes) and during (25 episodes) continuous intracoronary infusion of streptokinase. In addition, ST-segment monitoring demonstrated 12 episodes of spontaneous transient return of the ST segment to the base line in eight patients between the time of admission and the performance of coronary arteriography. During arteriographically documented reocclusion, intracoronary isosorbide dinitrate (2 mg) reestablished the patency of the coronary artery within one to two minutes in 14 of 28 episodes that occurred in 11 of 16 patients. After streptokinase infusion, intracoronary administration of isosorbide dinitrate was followed by dilatation of the infarct-related stenosis from a mean value (+/- SD) of 1.12 +/- 0.3 mm (58.1 +/- 12.1 percent) to 1.33 +/- 0.4 mm (51.6 +/- 12.9 percent; P = 0.004). Spontaneous intermittent coronary recanalization and reocclusion resulting from a variable combination of thrombosis and vasoconstriction are frequent during the early phase of acute myocardial infarction. We propose that the combination of intracoronary streptokinase and isosorbide dinitrate may increase the rate of stable coronary recanalization.