Aim To investigate the effects of hypoxic or ischemic preconditioning on cardiomyocytes. Methods In the hypoxia/reoxygenation (H/R) model of cultured neonatal rat cardi omyocytes, and the ischemia/reperfusion (I/S) model of the isolated or in situ rat hearts, the present study observes the effects of PC on H/R or I/R injury on cardiomyocytes. The infarct size, cell viability, LDH release and the content of MDA were measured. Results Hypoxia PC attenuated H/R injuries on cardiomyocytes. On the model of cultured neonatal rat cardiomyocytes, compared with the cardiomyocytes unpreconditioned, the number of viable cell and the SOD content were increased (P<0.01), the LDH release and the MDA content were decreased (P<0.01) in preconditioned group. In isolated or in situ rat hearts, ischemic PC decreased the injuries to cardiomyocytes. The coronary artery effusion or plasma LDH levels, tissue MDA contents, the infarct sizes and arrhythmia during occlusion or reperfusion were greatly decreased in preconditioned myocardium after a long time I/R than those in the unpreconditioned (P< 0.01). Conclusions Cultured neonatal rat cardiomyocytes, isolated and in situ rat hearts have PC phenomenon. PC can obviously protect cultured neonatal rat cardiomyocytes, isolated or in situ rat heart from H/R or I/R injuries.