Abstract: Objective: To investigate the effects of endomorphin-1(EM-1) postconditioning in rat with myocardial ischemia-reperfusion injury (IRI) in vivo and to assess its mechanism.Methods: twenty-four male Sprague Dawley rats were randomly divided into 4 groups:sham group(S group),ischemia-reperfusion group(I/R group), ischemia postconditioning group (IPO group),EM-1 postconditioning group(EM group).S group was established through threading under the left anterior descending branch of coronary artery and no occluding for 150 min;I/R group was established through occluding the left anterior descending branch of coronary artery for 30 min and reperfusion for 120 min;IPO group and EM group were respectively intravenous injection 0.9% NaCl 0.5ml and EM-1 20 μg/kg on 5 min before reperfusion,then reperfusion for 120 min.Dynamic index of blood flow was recorded and analyzed. At the end of reperfusion, arterial blood sample was obtained to measure plasma contents of malondialdehyde(MDA) and the activities of superoxide dismutase(SOD);the rats were sacrificed for assessment cell morphology of light microscopy. Results: Compared to S group,the heart rate,mean arterial pressure and rate-pressure product were decreased in I/R group(P<0.05~P<0.01);the contents of MDA was significantly increased and the activities of SOD was significantly decreased in I/R group(P<0.01);in cell morphology of light microscopy,the damage of myocardial structure was significantly increased in I/R group.The heart rate,mean arterial pressure and rate-pressure product in IPO and EM group were increased compared to I/R group except of MAP at 120 min following reperfusion in EM group(P<0.05~P<0.01),the contents of MDA in IPO and EM group was decreased and the activities of SOD was increased compared to I/R group(P<0.01),in cell morphology of light microscopy,the damage of myocardial structure was significantly reduced in IPO and EM group.Conclusions: EM-1 postconditioning can relieve ischemia reperfusion injury in myocardial protection,EM-1 postconditioning produces the protective effect by reducing the MDA and increasing the SOD.