It is not clear why late reperfusion therapy in patients with acute myocardial infarction is effective. An investigation was carried out as to whether or not reperfusion conducted 12 h after coronary occlusion causes myocardial salvage in dogs. Coronary arteries were occluded in 11 mongrel dogs and a portion of the occlusion (late reperfusion area; LR area) reperfused 12 h later; the other part was left occluded (permanent occlusion area; PO area). The dogs were maintained for 4 weeks after reperfusion. Regional myocardial blood flow (Q(m)) was measured by the non-radioactive colored microsphere method. In both areas, the transmurality of necrosis was measured by triphenyl tetrazolium chloride staining, and the amount of viable myocardium and the extent of fibrosis was determined by Azan-Mallory staining. Q(m) decreased markedly after coronary occlusion to similar levels in both areas until 12 h. Q(m) transiently increased in the LR area only following reperfusion after 12 h. The transmurality of necrosis in the PO area was 83.8 +/- 10.5%, but that in the LR area was 58.7 +/- 21.3%, a significant decrease (p<0.01). In the outer layer, the amount of viable myocardium was significantly greater, and the extent of myocardial fibrosis was significantly less in the LR area. Evaluation in the same heart of dogs confirmed the myocardial salvage effects of late reperfusion (12 h after coronary occlusion).