Abstract: To study the mechanism of protective effect of dexamethasone on myocardial reperfu-sion injury,the isolated working heart models were prepared in 30 SD rats,and then divid-ed into 3 groups：normal control group （gruop 1）, ischemia and reperfusion control group （group 2） and experimental group （group 3）. Cold crystalloid cardioplegic solution was perfused into the aorta of group 2 and 3 before cardiac arrest.Dexamethasone was added to the cardioplegic solution in group 3 at the concentration of 20 μmol/L. The levels of nitric oxide （NO） and creatine phosphokinase （CPK） in the blood of coronary sinus,and of NO synthase （NOS） and malondialdehyde （MDA） in myocardium were measured before arrest and 30 min after coronary reperfusion. The cardiac function was assessed at the same cor-responding times. The results showed that in group 2,as compared with group1,the levels of NO and constitutive NOS （cNOS） were reduced and inducible NOS （iNOS） increased after cardiac reperfusion，in the experimental group,as compared with group 2，the levels of NO and iNOS were reduced, while cNOS not significantly changed,and the recovery of cardiac function was ameliorated，the levels of MDA and CPK were reduced. These indicated that dexamethasone was helpful for myocardial protection. The reduction of iNOS and NO induced by dexamethasone may play an important role in myocardial protection.

Key words: Dexamethasone, Reperfusion injury, Myocardial protection, Isolated working heart, Rat