Abstract: The present experiment was performed to study the effect of postischemic reperfusion on infarct size in dogs. The results showed that no necrotic area was revealed on the sliced myocardium incubated with NBT in the group of coronary occlusion for 60 min. The infarct size expres sed as a percentage of the left ventricular and septal mass (g) was 0±0%(Mean SE). After post-isehemie reperfusion, the value was 11.05±4.28％ (P<0.01). In the group of coronary occlusion for 90 min, the percentage was 17.96±2.85％, and it was 21.41±1.86％ (P<0.05) followed by reperfusion for 120 min.In conclusion,early reperfusion causes enlarged infarct size and exacerbates the pre-existing ischemic myocardial injury.