The effectiveness of 24-hour hypothermic machine perfusion with TP-V (a hyperosmolar colloid solution containing dextrose, sucrose and ATP-MgCl2) alone, or in combination with oxygen free radical scavengers, was evaluated in isolated-perfused canine heart-lungs. Heart-lungs were perfused at 4 degrees C in either TP-V (n = 6), TP-V/Allopurinol (500 mg/L) (n = 6), or TP-V/Allopurinol (500 mg/L) & Catalase (5000 U/L) (n = 5). Lung inflation was maintained with 100% nitrogen. Following preservation, the heart-lungs were perfused with an albumin-mannitol perfusate for 3 hours at 37 degrees C, for functional, hemodynamic, and laboratory determinations. Cold preservation with TP-V/Allopurinol, and TP-V/Allopurinol & Catalase resulted in physiologically normal LDH levels during the 3-hour normothermic isolated perfusion test period. Significantly lower enzyme activity for CPK was evident at 0 (p less than .005) and 3 hours (p less than .05) of perfusion, while no significant differences in lactate production were seen among the groups. In addition, pH, PCO2, PO2, and left ventricular, aortic, and coronary artery pressures all remained within normal physiologic range, with no significant differences seen among the three groups. 99m Technetium scans demonstrated adequate patency among the heart-lungs, with better flow seen in those perfused with TP-V/Allopurinol & Catalase. Histological specimens confirmed a decrease in myocardial and pulmonary damage when Allopurinol and/or Catalase was used. It appears that oxygen free radical scavengers provide some protection from canine heart-lungs which have been hypothermically preserved for 24 hours.