Systemic artery-pulmonary artery shunts have been made in dogs and the relation between pulmonary artery pressure and blood flow and the development of vascular lesions has been studied. Arteriolar medial hypertrophy and intimal proliferation are not necessarily incited by a large blood flow, but such lesions are more readily induced by an end-to-end than by an end-to-side type of shunt. Intimal proliferation is not associated with stainable alterations in elastic tissue or lipid infiltration. Vascular lesions do not readily regress when the inciting factor is removed.