A donor lung is injured during preservation and is generally thought to be further injured by reperfusion on transplantation. Donor lungs from 15 adult male Lewis rats preserved by flush perfusion with cold Marshall's solution at 4 degrees C were examined by scanning and by quantitative transmission electron microscopy after 2, 4, or 7 h of storage at 4 degrees C and after transplantation (syngeneic) at 4 or 12 h (six animals per time interval). During preservation of the donor lung, capillary morphology changed rapidly. Both endothelial cells and type I pneumonocytes thinned (surface/volume ratio increased by 2 h in both; P less than 0.001). Pericapillary edema developed involving the blood-gas barrier. Basement membrane thickness increased significantly (P less than 0.001). Occasional breakage of the endothelial cell sheet occurred after 4 h of preservation, but even after 7 h of preservation there was no evidence of irreversible cell damage. The lamellar bodies of type II pneumocytes aggregated. Changes increased in severity with increase in preservation time. After transplantation, type I and type II pneumonocytes recovered after 12 h, but it took longer for the endothelial cell morphology to recover. Edema decreased rapidly during the first 4 h, despite the number of adherent neutrophils increasing 3-fold. The pulmonary capillaries of the transplanted lung showed no structural evidence of additional reperfusion injury, indicating a satisfactory method of preservation.