The thrombolytic effects and changes in left ventricular function after intravenous administration of nasaruplase were examined in a canine thrombus model of acute myocardial infarction. When 8 U/kg/min of nasaruplase was intravenously administered after 30 min of coronary arterial occlusion, coronary recanalization was achieved in 78.6% (11/14) of the animals with little change in plasma fibrinogen concentration. There was no further decrease in cardiac output, left ventricular ejection fraction or regional wall motion immediately after reperfusion, compared with 30 min after coronary arterial occlusion. However, all these parameters returned to pre-occlusion levels one week after reperfusion. Nasaruplase markedly reduced infarct size as well as cardiac hypertrophy following coronary arterial occlusion, demonstrating suppression of the development of ischemic myocardial damage. These results indicate that coronary thrombolytic therapy with intravenous administration of nasaruplase is useful for the treatment of acute myocardial infarction.