Abstract
The rapid injection of acetycholine into the pulmonary artery or right ventricle caused a transient fall of the pulmonary arterial pressure in 18 of 47 patients with various cardiovascular abnormalities. The response to acetylcholine was related to the initial level of pulmonary arterial pressure and probably due to vasodilation in the periphery of the pulmonary arterial tree. The response was found most frequently in individuals with a moderate rise of pulmonary arterial pressure. These results are related to the structural alterations that occur in the small pulmonary arteries in association with pulmonary hypertension.