Abstract: Ischemic preconditioning is the intrinsic and most potently myocardial protection we know now. To illu-minate its mechanism is the foundation of rational theraputic application. Nowadays there are some theories about delayed phase of preconditioning such as nitric oxide hypothesis, free radical mechanisms, protective protein syn-thesis and opening of AT P-sensitive potassium channels. By incorporation ³H-leucine, using liquid scintillation counter, we now know there was protective protein synthesis during preconditioning. SPECT could study the char-acteristics of preconditioned myocardium in vivo, and PET might further show the metabolism, energy consumption and its relationship to myocardium dysfunction.