Responses to vagal stimulation and acetylcholine were investigated in the feline pulmonary vascular bed under conditions of controlled pulmonary blood flow and constant left atrial pressure. Under baseline conditions, electrical stimulation of vagal efferent fibers increases lobar arterial pressure. However, when vasoconstrictor tone was increased, a depressor response was unmasked. The pressor response under baseline conditions and the depressor response under enhanced tone conditions were blocked by phenoxybenzamine and atropine. These data suggest that, in the cat, the vagus is composed of efferent fibers from both the sympathetic and parasympathetic systems. After treatment with 6-hydroxydopamine to destroy the integrity of the sympathetic system, vagal stimulation caused significant frequency-dependent decreases in lobar arterial pressure when lobar vascular tone was increased by infusion of a stable prostaglandin endoperoxide analog or ventilatory hypoxia. Injections of acetylcholine also caused significant dose-related decreases in lobar arterial pressure when lobar vascular resistance was elevated. Depressor responses to vagal stimulation and acetylcholine in 6-hydroxydopamine-treated animals were blocked by atropine and enhanced by physostigmine. Decreases in lobar arterial pressure in response to vagal stimulation in 6-hydroxydopamine-treated animals with enhanced tone were blocked by hexamethonium, whereas responses to injected acetylcholine were not altered by the ganglionic blocking agent. Decreases in lobar arterial pressure in response to vagal stimulation and acetylcholine were similar when the lung was ventilated and when the left lower lobe bronchus was obstructed. In addition, responses to vagal stimulation were similar when systemic arterial pressure was decreased to the level of pressure in the perfused lobar artery. Responses to acetylcholine were not altered after treatment with 5,8,11,14-eicosatetraynoic acid, a lipoxygenase inhibitor. The present data 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.