This study was designed to test the hypothesis that infusion of ATP-MgCl2 during reperfusion following a prolonged period of hypothermic global ischemia would result in enhanced functional recovery of cardiac function. Two groups of dogs (n = 6 each) were placed on cardiopulmonary bypass (CP) with systemic hypothermia to 28°C and subjected to 150 min of aortic cross-clamping. Crystalloid cardioplegia was infused every 20 min during ischemia. Reperfusion and rewarming were carried out for 20 min before discontinuation of CP bypass. During reperfusion, the experimental group received ATP-MgCl2 (1.0 mg/kg/min ATP, 0.33 mg/kg/min magnesium). At 15 and 45 min following bypass, hemodynamic assessment was carried out for each animal by constructing Starling curves over a range of filling pressures at constant heart rate and comparing each animal to its own prebypass control level. The results indicated that ATP-treated animals exhibited complete functional recovery whereas control animals showed marked reduction in hemodynamic performance and myocardial compliance and had a higher myocardial water content (P < 0.05). We conclude that infusion of ATP-MgCl2 during reperfusion following hypothermic ischemia may help ameliorate reperfusion injury. © 1987.