Abstract： The aim of the present study was to investigate the protective effect of polydatin against myocardial ischemia/repeffusioninjury in rats and the underlying mechanism. In anesthetized rats, ischemia and reperfusion arrhythmia produced by ligating and loosingthe coronary artery was recorded and myocardial infarct size was measured. In Langendorff isolated rat heart, cardiac function wasrecorded before and after 30 min of global ischemia followed by 60 rain of reperfusion. The parameters of cardiac function include leftventricular developed pressure (LVDP), maximal differentials of LVDP (±LVdp/dtmax) and coronary flow (CF) were measured. Myocar-dial superoxide dismutase (SOD) activity, the contents of myocardial malondialdehyde (MDA) and nitric oxide (NO) as well as theactivity of nitric oxide synthase (NOS) were measured in isolated heart. The results showed: (1) Axrhythmia score and myocardialinfarct size were significantly lower in polydatin group than that in the control group (P<0.05, P<0.01); (2) The recovery of LVDE±LVdp/dtmax and CF during reperfusion in polydatin group were significantly better than that in the control rats (P<0.05, P<0.01); (3)SOD activity in polydatin group was significantly higher than that in the control group, but MDA content was lower in polydatingroup than that in the control group (P<0.05); (4) NO content and NOS activity, especially constitutive nitric oxide synthase (cNOS)activity in polydatin group were higher than that in the control group (P<0.05); (5) L-NAME, the NOS inhibitor, reversed theprotective effect of polydatin against ischemia/reperfusion injury. The results suggest that polydatin has a protective effect againstischemia/reperfusion injury in rat heart. The cardioprotection of polydatin is mainly mediated by cNOS which leading to an increase inNO production.