The left lung was transplanted in 12 adult beagles; the donor lung had been preserved by flush perfusion of the pulmonary artery with 60 ml/kg of Euro-Collins solution at 4°C then stored in Euro-Collins solution at 4°C for 6 hours. Biopsy specimens were taken from control and donor left lungs before and immediately after perfusion, after 6 hours storage, and after transplantation at 4 and 24 hours (total = 46). Tissue from seven animals was injected with the tracer ruthenium red. All tissue was examined by light and electron microscopy. After preservation capillary edema developed, but ruthenium red did not penetrate junction complexes, indicating structurally intact complexes. The proportion of ruthenium red-labeled plasmalemmal vesicles decreased (p < 0.05). Endothelial cells and type I pneumonocytes became abnormally thin and detached from the basement membrane. The epithelial, but not the endothelial, sheet was disrupted. Four hours after transplantation, edema decreased, and endothelial and epithelial cells regained a more normal shape. The proportion of labeled plasmalemmal vesicles increased (p < 0.05) by 24 hours, when the value was similar to normal. During preservation, edema formed when alveolar capillary white cell count was normal and decreased after reperfusion when the cells increased threefold (p < 0.001), indicating that edema was not neutrophil dependent. Many small muscular arteries constricted intensely on cold perfusion. Four hours after transplantation, these abnormalities had improved. Conclusion: The pulmonary vasculature showed no evidence that it had sustained additional structural injury after reperfusion. Reperfusion was associated with a gradual resolution of the structural abnormalities incurred during preservation.