Abstract: Objective: To examine the effects of different concentrations of nitroglycerin (GTN) on cardiac ischemia and reperfusion injury in isolated perfused rat hearts.Methods: Male Sprague-Dawley rats were used for Langendoff isolated heart perfusion.The hearts were subjected to 30 min regional ischemia (occlusion of left anterior descending artery) and 120 min reperfusion.The ventricular hemodynamic parameters,lactate dehydrogenase (LDH) release during reperfusion and myocardial infarct size were measured.Results: Administration of GTN at high concentration (2×10^-6 mol/L) significantly aggravated post-ischemic myocardial injury characterized by depressed cardiac function recovery (P<0.05-P<0.01),enlarged myocardial infarct size (P<0.01) and increased LDH release (P<0.05).GTN at middle concentration (1×10^-7 mol/L) did not change the cardiac function recovery,myocardial infarct size and LDH release (P<0.01).GTN at low concentration (1×10^-8 mol/L) further improved the cardiac function recovery,reduced infarct size and LDH release (P<0.05-P<0.01).Conclusions: GTN at high concentration can aggravate cardiac ischemia and reperfusion injury,GTN at middle concentration do not influence heart while GTN at low concentration can protect heart.