Generation of free radicals upon reperfusion has been cited as one of the major causes of ischaemia/reperfusion injury. The following series of experiments was designed to study the effect of manganese superoxide dismutase (MnSOD) overexpression in transgenic mice on ischemia/reperfusion injury. A species of 1.4 kb human MnSOD mRNA was expressed, and a 325% increase in MnSOD activity was detected in the hearts of transgenic mice with no changes in the other antioxidant enzymes or heat shock proteins. Immunocytochemical study indicated an increased labeling of MnSOD mainly in the heart mitochondria of the transgenic mice. When these hearts were perfused as Langendorff preparations for 45 min after 35 min of global ischemia, the functional recovery of the hearts, expressed as heart rate×left ventricular developed pressure, was 52±4% in the transgenic hearts as compared to 31±4% in the non-transgenic hearts. This protection was accompanied by a significant decrease in lactate dehydrogenase release from the transgenic hearts. Overexpression of MnSOD limited the infarct sizein vivoin a left coronary artery ligation model. Our results demonstrate that overexpression of MnSOD renders the heart more resistant to ischemia/reperfusion injury.