After acute coronary occlusion in primates, the time period during which reperfusion results in significant salvage of reversibly injured myocardium was investigated. In 23 monkeys, the left anterior descending coronary artery was occluded from 1 to 6 h; and in 5 others, occlusion was maintained for the 1-wk study. Unipolar epicardial electrocardiograms were monitored from mapping points on the anterior and lateral left venticle. S-T segment elevation (S-T upward arrow) and R + S wave amplitude (RS) were measured before occlusion and at regular intervals during occlusion and reperfusion. Summated S-T upward arrow (SigmaS-T upward arrow) and summated RS (SigmaRS), computed for mapping points demonstrating greater than 2 mV S-T upward arrow, were used as serial measures of electrical injury. SigmaS-T upward arrow peaked within 2-h postocclusion and then gradually declined throughout the period of occlusion suggesting the progress of infarction within the area of injury. After reperfusion SigmaS-T upward arrow rapidly declined to near cnotrol values indicating the extent of reversible injury. During the period of occlusion, the magnitude of voltage loss in SigmaS-T upward arrow as a percent of maximum SigmaS-T upward arrow was proportional to the duration of occlusion, though the rate of loss decreased with increasing time of occlusion. Reperfusion after 6 h of occlusion resulted in reversal of only a small remaining component of the maximum current of injury. The voltage decrease in SigmaRS (from control values) was proportional to the duration of occlusion, though the decrease was accelerated during the first 2-h postocclusion. Whereas reperfusion interrupted the decline in SigmaRS, a consistent increase in SigmaRS postreperfusion was observed only after occlusion of 1 h. With respect to reperfusion groups, significance in SigmaS-T upward arrow voltage loss as a percent of maximum SigmaS-T upward arrow was demonstrated between 2-h and 4-h, 4- and 6-h, and 6-h and chronically ligated animals. Significance in SigmaRS voltage loss as a percent of control SigmaRS was demonstrated between 2- and 4-h, and 4- and 6-h reperfusion groups. Hearts were excised at 7 days for histological assessment of infarct size. Planimetric determination of left ventricular areas and areas of necrosis using slides made from 10 serial cross sections were used in estimating the percent of left ventricle infarcted. A significant reduction in infarct size was demonstrated between reperfused animals at 2 h and the 4- and 6-h reperfusion groups. A trend was noted suggesting increasing infarct size up to 6 h after experimental occlusion.