Mechanisms are described that are known to prolong survival of ischemic myocardium. Some of these mechanisms are species-specific: collateral blood flow does not contribute to survival in rats, rabbits, and pigs but it salvages subepicardial myocardium in the dog and probably in man. The most important defense mechanism is enlargement of collaterals by growth of pre-existing smaller vessels. This process is not operative in sudden coronary occlusion but may save a substantial portion of the myocardium if occlusion occurs more slowly, i.e. several days up to a week. Experimental evidence is presented that thrombotic occlusion, clot-retraction, and partial clot lysis by endothelium can be sufficiently dynamic to allow intermittent myocardial perfusion and to permit collaterals to grow. The glycogen stores of the heart are of limited importance. They are useful for glycolytic ATP-production, the limitation is imposed by the inhibition of glycolysis in ischemia due to unfavourable pH and lack of NAD. Some beta-blockers do interfere with glycogenolysis in the heart. The cardioprotective role of adenosine needs further study. An interesting concept is the change in resistance to ischemia after repeated cycles of ischemia. It is not known at present whether repeated cycles will increase or decrease the myocyte's resistance against ischemia.