Heparin in large doses significantly improved epicardial electrocardiographic findings and preserved myocardial tissue and creatine phosphokinase (CPK) after coronary ligation in the dog. Epicardial S-T segment elevation 15 minutes after occlusion was lowered 84% (from 64.5 + 8.5 [standard error of the mean] to 10.4 +/- 3.0 mv) by heparin infusions of 60,000 units. Myocardial creatine phosphokinase depletion was reduced from 39 to 24% at comparable levels of S-T segment elevation. Histologic evidence of necrosis decreased 32%. It is concluded that heparin can reduce the extent of ischemic injury after acute coronary occlusion in the dog. These results may lend insight into the factors responsible for ischemic injury.