The purpose of this study was to investigate the hypothesis that balloon distention of the main pulmonary artery (MPA) induces pulmonary hypertension that is produced by a neural reflex and to investigate the possible efferent components of its reflex arc. Using a specially designed triple-lumen balloon catheter, positioned under fluoroscopy in the MPA, the hemodynamic responses to MPA distention were studied before and after the following: surgical denervation of the bifurcation of the MPA, chemical sympathectomy (6-hydroxydopamine), 100 percent oxygen breathing, and vagotomy. Our findings suggest that the experimental pulmonary hypertension caused by distention of the MPA is produced by excitation of stretch receptors in or near the bifurcation of the MPA and that the efferent limb of this reflex is predominantly mediated via the adrenergic nervous system.