It has been reported that tracheal smooth muscle of guinea pig is innervated by both the adrenergic and nonadrenergic noncholinergic (NANC) inhibitory nervous system. However, NANC inhibitory nerve supply to the lower airway of guinea pig has not yet been demonstrated in in vivo experiments. We performed the present study to evaluate the physiological role of NANC inhibitory nerves in tracheal smooth muscle of guinea pig in vitro, and in anesthetized guinea pigs in vivo. Innervation of NANC inhibitory nerves to tracheal smooth muscle was much greater than that of adrenergic nerves (77.8 +/- 3.8%, 22.2 +/- 3.8%, respectively, p less than 0.01). Neither decrease in RL nor increase in CL, however, was observed with vagal stimulation during serotonin infusion after the administration of propranolol. The role of NANC inhibitory nerves in histamine-induced bronchoconstriction (HIB) was investigated. HIB was enhanced by vagotomy in guinea pig pretreated with propranolol (p less than 0.01). These results suggest the role of the NANC inhibitory nervous system in the attenuation of HIB in vivo in the guinea pig.