AbstractThe gross and microscopic innervation of the bifurcation and extrapulmonary portion of the pulmonary artery in the adult cat is described. Morphopharmacologic studies indicated that the major efferent and afferent innervation is contributed by the right recurrent cardiac nerve, a branch of the recurrent laryngeal.A morphologic description of the adventitial nerve bundles, preterminal axon plexus and medial, fine unmyelinated neurites as seen in methylene blue and Bielschowsky silver preparations is given. Rare, classical, pressoreceptor arborizations of de Castro type II were identified, but evidence was presented of degenerative and regenerative neural activity in the adventitial and pre‐terminal plexus manifested by irregular axonic swellings, myelin ellipsoids with vacuolation, terminal growth cones and collateral sprouting. In these degenerative zones, the plexus assumed a more discontinuous organization showing features very suggestive of a simple pressoreceptor system composed predominantly of de Castro type I terminations.Thein vitropharmacologic studies using the isolated pulmonary artery‐nerve preparation confirmed the existence of effector, vasoconstrictor adrenergic sympathetic fibers in the recurrent cardiac nerve. Small unmyelinated perivasal and perivasal collateral fibers disappear or show severe degenerative changes following bilateral sympathectomy, and are considered to represent the sympathtic autonomic compon