The purpose of this study was to investigate the time during which the emergency reconstruction of the occluded coronary artery is most effective. This problem was examined from the viewpoint of the extent of reversibility of the damage to ischemic myocardial muscle, by using forty nine mongrel dogs. The surgical reconstruction of the occluded coronary artery within 6 hours after the onset of myocardial infarction was most effective. In such cases, the extent of myocardial infarction remained small in size, and the healing of necrotic myocardial muscle was hastened. Thus, the development of cardiac failure was well protected.