Abstract: Objective: To investigate the role of nitric oxide(NO) in reduction of myocardial ischemia-reperfusion(I /r) injury by sufentanil postconditioning in cardiac valve replacement patients under cardiac pulmonary bypass(CPB) . Methods: Sixty patients(ASA grade Ⅱ or Ⅲ，NYHA class Ⅱ or Ⅲ) of both sexes, aged 19-64, scheduled for cardiac valve replacement were randomly divided into 4 groups(n = 15 each): control group(group C) , sufentanil 0. 5 g /kg group(group S1 ) , sufentanil 1. 0 g /kg group(group S2 ) and sufentanil 2. 0 g /kg group(group S3 ) . In groups S1 ，S2 and S3 , sufentanil 0. 5，1 . 0 and 2. 0 g /kg were infused over 2 min via aortic root 5 min before aortic unclamping respectively. In group C, the equal volume of normal saline (2 ml /kg) was infused instead of sufentanil. Blood samples were taken from radial artery before induction of anesthesia(T0 ) and at 2 h(T1 ) ，4 h(T2 ) ，8 h(T3 ) , 24 h (T4 ) and 48 h (T5 ) after aortic unclamping for determination of plasma concentrations of cardiac troponin-I (cTnI) and NO and activities of creatine kinase isoenzyme-MB(CK-MB) and nitric oxide synthase(NOS) . The hemodynamic parameters，duration of CPB, time of aortic clamping, extubation time，duration of stay in ICU, and myocardial contractility score and volume of drainage at 24 h after the operation were recorded. The restoration of spontaneous heart beat were observed. results: The plasma cTnI concentrations and CKMB activity were significantly lower，while the plasma NO concentrations and NOS activity were significantly higher at T1-3 in group S1 than that in group C(P ＜ 0. 01) . The plasma cTnI concentration and CK-MB activity were significantly lower at T1-5 , the plasma NO significantly higher at T3-4 , and the myocardial contractility score at 24 h after the operation was significantly lower in groups S2 ，3 than that in group S1 (P ＜ 0. 05 to P ＜ 0. 01) . There was no significant difference between groups S2 and S3 (P ＞ 0. 05) . Conclusions: Sufentanil postconditioning can relieve myocardial I /r injury in patients undergoing cardiac valve replacement under CPB by activating the NOS /NO pathway，which is dose-dependent and ceiling-effective.