Abstract： Objective To study the effect of growth hormone preconditioning on apoptosis and energy me tabolism in mrs with experimental myocardial ischemia reperfusion injury.Methods Forty-two rats were divided randomly into three groups:sham operated group(n=14,sham operated for 24h),IR group(n=14) and GH group(n=14).30 min ischemiafollowing by 24h reperfusion were performed in IR group and GH group.The rata in GH group were treated with subcutaneous injection of GH 0.1 U/kg body weisht per day for 7d and seventh injection was administered before the surgery.Each rat from otller groups was treated with subcutaneous injection of normal saline 0.5 ml per day.Eight rats were randomly taken from each group and terrainal deoxynueleotidyl trarm ferase mediated dUTP fluoreseein nick and labeling(TUNEL)was used to detect the changes of apoptosis.Five rats were randomly taken from each group and ATP of ischemic myocardium was measured by high-performance liquid chromatography(HPLC).Histopathology and electronic ultrastrueture of myocardial tissue were observed.Remrs Apoptosis index(AI) and depletion of ATP increased remarkably in IR group than that of sham operated group 24h after reperfusion (P＜0.05).111e rein/irkabie changes of myocardial histopathology and ultrastructure were ob served in IR group.Compared to IR group.AI and depletion of ATP in GH group were better than that of IR group 24h after reperfusion(P＜0.05),and the changes of myocardial histopathology and uhrastrueture were alleviated in GH group.Conclusion The AI and depletion of ATP were lessened due to usage of GH which indieates that GH can protect myocardium after IR.