Morphological changes in lung tissue of heterotopic heart-lung transplants have been studied 28 days following transplantation in 20 rats. Lung tissue showed disperse haemorrhagic necrosis often sparing bronchi and branches of pulmonary artery and conspicuous accumulation of bronchial secretion. Partial disappearance of cardiac muscle cells from the walls of pulmonary veins has been ascribed to tissue hypoxia.