Objective: To investigate the protective effects of preconditioning morphine on rabbit myocardium during ischemia-reperfusion. Methods: Thirty New Zealand male white rabbits were randomly assigned to three groups:control,I/R and morphine groups.In morphine group 1.0 mg/kg morphine was given preoperationaly,in control and I/R groups 1.0 ml/kg NS was given.Twenty-four hours later rabbits in morphine and I/R groups underwent 40 min of coronary occlusion followed by 2 hours of reperfusion;for control group only sham operation was performed.At the end of the reperfusion,infarct size (IS) and area at risk (AAR) were defined by Evans blue and TTC staining.At the end of the reperfusion blood samples were taken for determination of plasma SOD activity and MDA levels.The heart was harvested and levels of the HSP27 were determined by Western blot,and the heart ultrastructures were observed under the electron microscopy. Results: Compared with I/R group,morphine significantly reduced infarct size (21.5%±2.4% vs 37.8%±1.7%,P<0.05).The morphine had a lower level of MDA and higher levels of SOD and HSP27 than those in I/R. Conclusion: Preconditioning of morphine demonstrates cardioprotective effect on ischemia/reperfusion injury,which may be associated with increased HSP27 levels in the heart.