Bronchiolar morphology after systemic arterial interruption. To assess ischemic lesions as a factor in obliterative bronchiolitis after lung transplantation, the authors severed the left bronchial arteries of 15 dogs, together with the left stem bronchus, the latter being immediately reanostomosed. They examined the bronchioles at weekly interfals up to three and a half months. On the week chosen each dog was anesthetized, totally heparinized, and exsanguino-perfused with saline. Just after heart arrest, the thoracic aorta was injected with a barium solution until this white medium appeared in the bronchial arteries. The heart-lung blocs were excised en bloc, submitted to soft-tissue x ray, fixed, and then sliced to 1 cm. Corresponding right and left 5-mm-thick samples of these slices were prepared for contact microradiography followed by histologic 5-to-20-micron-thick, stained, correlated specimens. For two weeks the left bronchial arteries remained empty, but there was no necrosis or edema. Between two and four weeks barium solution appeared in the bronchial arteries, and the bronchiolar epithelium had become multistratified. Later the left bronchiangiogram became similar to the right, but there were more folds of the mucosa and a little submucosal fibrosis. These studies provide proof that no significant ischemic lesions occurred during repermeation of the bronchiolar vascular bed. Ischemia, if existent, is not a significant factor in obliterative bronchiolitis. 