Abstract: AIM:To investigate the protective effect of melatonin (Mel) on myocardial ischemia/reperfusion (MI/R) injury and its underlying mechanisms. METHODS: Eighty male Sprague Dawley rats (200-250 g) were subjected to myocardial ischemia/reperfusion (MI/R, I 30 min, R 6 h) and randomly divided into four groups: MI/R+V (absolute alcohol diluted in sterile saline to 0.1 M, 1 ml/d, 7 d before MI/R; 1 ml/d, 15 min before reperfusion, ip), MI/R+Mel ［(10 mg/(kg·d), 7 d before MI/R; 15 mg/(kg·d), 15 min before reperfusion］, MI/R+Mel+EX ［Mel treatment following the same routine; EX527, 5 mg/(kg·d), every other day for three times; 5 mg/(kg·d), 20 min before reperfusion］ and sham (rats undergoing the same surgical procedures without tying the suture). Cardiac function 72 h after reperfusion, serum CK and LDH level, apoptotic index, infarct size, SIRT1 expression, Ac-Foxo1 expression and apoptosis related protein expression was detected. RESULTS: Seventy two h after MI/R operation, melatonin treatment group showed improved cardiac function, lower serum CK and LDH levels, decreased apoptotic index, decreased infarct size, upregulated SIRT1 expression, downregulated Ac-Foxo1 expression and decreased apoptosis related protein expression (all P<0.01). Moreover, SIRT1 inhibitor EX527 downregulated SIRT1 expression, upregulated Ac-Foxo1 expression and blocked the cardioprotective effect of melatonin. CONCLUSION: These data indicate that melatonin treatment attenuates myocardial ischemia/reperfusion injury by reducing apoptosis via a SIRT1 dependent signaling pathway.