Intracoronary Urokinase administration was attempted in 42 patients with acute myocardial infarction. One patient died before Urokinase infusion could be started, two patients during treatment. 34 patients (83%) had total occlusion of a coronary vessel, 7 patients (17%) had a subtotal occlusion. Urokinase was administered at a rate of 10.000 IU/min in 11 consecutive patients (Group I), 6000 IU/min in 11 patients (Group II), 4000 IU/min in 11 patients (Group III), 2000 IU/min in 8 patients (Group IV). Recanalization of the occluded vessel or amelioration of the subocclusion was obtained in 36 patients (88%). 35 patients were controlled 12 days after the procedure, with coronary-angiography and left ventriculography. Patency persisted in 31 patients and successful thrombolysis was associated with significant improvement of left ventricular ejection fraction in those patients with depressed ventricular function before Urokinase treatment.