Responses to vagal stimulation, acetylcholine, catecholamines, and sympathetic nerve stimulation were investigated in the feline pulmonary vascular bed under conditions of controlled pulmonary blood flow and constant left atrial pressure. Results of these studies suggest that, in the cat, the vagus is composed of efferent fibers from both the sympathetic and parasympathetic systems. The data also suggest that the feline pulmonary vascular bed is functionally innervated by cholinergic nerves, and that vagal stimulation dilates the pulmonary vascular bed by releasing acetylcholine, which acts on muscarinic receptors in pulmonary vessels. alpha and beta 2-adrenoceptors appear to be present in the feline pulmonary vascular bed, and both types of adrenergic receptors are innervated by the sympathetic nervous system. Selective alpha 1- and alpha 2-receptor agonists produce pulmonary vasoconstrictor responses, suggesting the presence of both alpha 1- and alpha 2-receptor subtypes in the feline pulmonary vascular bed. The pulmonary vascular response to norepinephrine, dopamine, and dobutamine is the algebraic summation of activation of alpha- and beta-adrenoceptors. Dopamine possesses greater alpha-(vasoconstrictor) activity when compared to dobutamine, whereas dobutamine has greater beta-(vasodilator) activity.