Reperfusion in man has been achieved by coronary by-pass surgery in the acute stage of infarction. Follow-up angiography revealed significant improvement of left ventricular function. Intracoronary streptokinase infusion resulted in recanalization of acute coronary occlusion in 80 percent of cases within 20-30 minutes of infusion time. Reperfusion achieved by intervention angiography has been associated with improved local wall motion as well as improved left ventricular function. Improvement in left ventricular function depended on the duration of symptoms prior to reperfusion and on the presence or absence of collateral blood flow. Available data suggested that improvement was more likely to occur if reperfusion was achieved within 3-4 hours. However, since the majority of patients reached the hospital after greater than 4 hours of chest pain, study of functional changes following late reperfusion was of great practical significance.