We have investigated the coronary vasodilator responses to nitrates in the early stages of acute myocardial infarction before, during and after the administration of thrombolytic therapy. Before thrombolysis, intracoronary doses (2-4 mg) of isosorbide dinitrate failed to induce recanalisation of the totally occluded infarct related artery. Episodes of coronary reopening and reocclusion associated with ST-segment resolution and re-elevation were frequently observed both spontaneously before and during thrombolytic therapy. In 50-86% of patients, intracoronary isosorbide dinitrate promptly re-established full coronary patency immediately after acute reocclusion or whenever coronary occlusion was incomplete, both before and during thrombolysis. After thrombolysis further intracoronary isosorbide dinitrate dilated the infarct related stenoses but not uninvolved adjacent normal coronary branches. Coronary thrombosis and constriction frequently interact during the early phases of acute myocardial infarction. The combination of high local concentrations of nitrates with thrombolytic agents promotes stable coronary recanalization and may be beneficial in patients with acute myocardial infarction.