〔Abstract〕 Objective To investigate the effect of morphine pretreatment on the expression level of Caspase-3 and apoptosis of myocardial cells after ischemia reperfusion injury in rats. Methods A total of 51 male adult SD rats were randomly divided into 3 groups: the control group(Sham), ischemia-reperfusion group(IR) and morphine preconditioning group(1 μmol/L, MP). The Sham group was perfused with K-H solution for 195 min. In IR group,K-H solution was infused for 45 min to prepare in vitro whole heart ischemia for 30 min,and 10,60 and120 min post-ischemia reperfusion models were made. In MP group,K-H solution with morphine was perfused and K-H solution without morphine was perfused for 5 min each before ischemia,with a total of 3 cycles. Infarct volume( IS),area at risk( AAR) and IS/AAR were determined by 2,3,5-triphenyl-tetrazolium( TTC) staining. Lactic dehydrogenase( LDH) activity in coronary artery outflow was measured by chemical colorimetry. Caspase-3 and cardiomyocyte apoptosis were detected after 10,60 and 120 min of reperfusion via Western blot and TUNEL staining,respectively. Results Compared with sham group,the percentage of infarct volume at the end of reperfusion increased in IR group,LDH activity,Caspase-3 protein expression and cardiomyocyte apoptosis ratio significantly increased in IR group at each time point after reperfusion( P<0. 05). Compared with IR group,morphine pretreatment significantly decreased IS/AAR ratio,LDH active Caspase-3 protein level and myocardial apoptosis ratio( P<0. 05). LDH level at 10 min reperfusion,Caspase-3 protein expression at 60 min reperfusion and apoptosis rate at120 min reperfusion were the highest in IR group and MP group. Conclusion The protective effect of morphine pretreatment on myocardium is associated with decreased expression of Caspase-3 protein at the early stage of reperfusion and decreased apoptosis of myocardial cells after reperfusion.