It has been shown that repeated brief coronary occlusions increase myocardial resistance towards prolonged episodes of ischemia. This phenomenon, which renders the heart more tolerant to ischemia with subsequent limitation of infarct size, has been termed ischemic preconditioning and has been described in a variety of species. Preconditioning may also protect the heart against postischemic dysfunction and ventricular arrhythmias. Although the beneficial effects seem to be transient, they re-appear at 24 hours, representing a "second window of protection." Ischemia-induced activation of adenosine receptors and opening of ATP-sensitive potassium channels appear to play a role in the acute cardioprotection. For the late protection, stress protein synthesis may play a role. There is experimental and clinical evidence that preconditioning effects may also exist in the human heart. If so, the mechanisms of ischemic preconditioning might be applied to future therapy.