En bloc transplantation of the heart and lungs was performed in 15 chacma baboons; the donor organs were stored between 4 and 6 hours before transplantation. The hearts were perfused in the donor animals with 15 ml/kg Wicomb's cardioplegic solution at 4 degrees C, the lungs with either 20 ml/kg 4 degrees C Collins' solution with an added 2.5% dextrose and 12 mEq magnesium sulfate (Collins' solution, group 1, n = 8), Collins' solution plus superoxide dismutase (40,000 U/L superoxide dismutase, group 2, n = 4), or Collins' solution plus superoxide dismutase plus peroxidase (5000 U/L peroxidase plus mannitol, group 3, n = 3). The pulmonary artery perfusion pressure was not allowed to exceed 50 cm water; the lungs were maintained at 30% to 50% inflation, and external cooling was applied. After explantation the thoracic organs were stored in 0.9% saline solution at 4 degrees C. In groups 1 (Collins' solution) and 2 (Collins' plus superoxide dismutase) all surviving baboons revealed normal blood gas values and normal light and electron microscopic histology at 24 hours. Three animals had further biopsies at intervals between 1 and 9 days, at which time the histology of the lungs proved normal and well preserved. All three baboons in group 3 (Collins' plus superoxide dismutase plus peroxidase) had grossly abnormal blood gas values from the time of operation, and all died within 9 hours; light microscopy of the lungs showed early lung infarctlike lesions and in one case pulmonary edema. These preclinical findings proved that storage of the lungs in Collins' solution with or without superoxide dismutase is possible for up to 5 hours; the addition of peroxidase had a detrimental effect.