Objective To investigate the cardioprotective effects of morphine on ischemic reperfused rat heart in vitro and its mechanism Methods The isolated rat heart was perfused in a Langendorff apparatus Infarct myocardium was determined by TTC Coronary flow (CF), heart rate (HR), left ventricular pressure (LVP), the first derivative of ventricular pressure (LVP/dtmax) and infarct size after ischemia and reperfusion in rat heart given 0.3 μmol/L morphine were observed The effects of naloxone and glibenclamide on the cardioprotection of morphine were also measured Results After ischemia and reperfusion, CF, HR, LVP and LVP/dtmax of isolated rat hearts decreased significantly ( P <0 01) After morphine preconditioning, HR, LVP and LVP/dtmax increased ( P <0 01) and infarct size was reduced significantly ( P <0 01), while no significant change in CF ( P >0 05) The cardioprotective effects of morphine were abolished by naloxone or glibenclamide completely Conclusions Morphine can reduce ischemia reperfusion injuries in isolated rat heart The cardioprotective effects of morphine are mediated by a local opioid receptor K ATP channel linked mechanism in rat hearts Objective To investigate the cardioprotective effects of morphine on ischemic reperfused rat heart in vitro and its mechanism Methods The isolated rat heart was perfused in a Langendorff apparatus Infarct myocardium was determined by TTC Coronary flow (CF), heart rate (HR), left ventricular pressure (LVP), the first derivative of ventricular pressure (LVP / dtmax) and infarct size after ischemia and reperfusion in rat heart given 0.3 μmol / L morphine were observed The effects of naloxone and glibenclamide on the cardioprotection of morphine were also measured Results After ischemia (P <0.01) and infarct size was reduced significantly (P <0.01) after reperfusion, CF, HR, LVP and LVP / dtmax of isolated rat hearts decreased significantly <0 01), while no significant change in CF (P> 0.05) The cardioprotective effects of morphine were abolished by naloxone or glibenclamide completely Conclusio ns Morphine can reduce ischemia reperfusion injuries in isolated rat heart The cardioprotective effects of morphine are mediated by a local opioid receptor K ATP channel linked mechanism in rat hearts