Vascular corrosion casting was used to search for angiogenesis in the blood vessels of the lungs of rats given monocrotaline. Animals treated with monocrotaline had new well-differentiated arteries and veins on their pleural surfaces. Animals not treated had no large vessel on their pleural surfaces. Animals receiving monocrotaline had capillaries around major arteries that were more dense, widened, and less tubular than normal. These capillaries occasionally occurred in sheets and had blind endings. The control animals had delicate, uniform, tubular capillaries. Alveolar capillaries in both groups showed no evidence of increase in size or number or change in shape. Light microscopy confirmed the finding of new vessels found with the casts. The finding of angiogenesis on the pleural surface and in the bronchovascular bundle, but not in the alveolar capillaries, suggests a basic difference in how these capillary beds respond to angiogenic stimuli. If alveolar capillaries are unable to undergo angiogenesis, concepts of lung development and tumor growth may be significantly altered. The lung may be a unique organ to study angiogenesis because of the different angiogenic potential of its two circulations. Study of these differences may lead to better understanding of inhibition of angiogenesis.