Background: Lazaroid, an inhibitor of iron-mediated lipid peroxidation, has been shown to reduce free radical-mediated injury after ischemia and reperfusion. The effect of Lazaroid U-74389G was investigated on ischemia-reperfusion injury of the heart through preservation and transplantation (Tx) in dogs. Methods: Eleven pairs of adult mongrel dogs weighing 8.5 to 12 kg formed the recipient-donor combinations. Following electromechanical arrest of the heart using cardioplegia, the coronary vascular bads were washed out with a cold University of Wisconsin solution followed by 12-hour preservation and orthotopic Tx. Experimental animals were divided into 2 groups; 6 pairs formed the control group, and 5 formed the Lazaroid-treated group in which Lazaroid U-74389G at 10 mg/kg was administered intravenously 30 minutes before reperfusion of the heart. The cardiac function including cardiac output, left ventricular (LV) pressure, and LV dp/dt was assessed 2 hours after Tx by comparing it with the recovery rates (%) from cardiac function of donor dogs. Each transplanted heart was harvested for histological study. Results: The recovery of cardiac function after Tx was significantly better in the Lazaroid-treated group than in the control group. Histologically, myocardial damage, evaluated by both light and transmission electron microscopy, was less evident in the Lazaroid-treated group than in the control group. Conclusion: Early cardiac function following Tx was significantly better and histological damage was less in the Lazaroid-treated group than in the control group, suggesting that Lazaroid U-74389G is effective in preventing ischemia-reperfusion injury after preservation and Tx.