Human isolated central (5 to 12 mm) and peripheral (< 2 mm) bronchi were contracted with 3 microM histamine. Relaxations were then evoked by electrical field stimulation (EFS) (1 to 32 Hz, 1 ms, 12 V for 15 s in the presence of indomethacin, atropine, and propranolol). The magnitude, time-course, and frequency-response relationship of these nonadrenergic, noncholinergic (NANC) relaxations were similar in the central and the peripheral airways. NG-Nitro-L-arginine (L-NOARG) (10 microM) inhibited the tetrodotoxin-sensitive NANC relaxations in both central and peripheral bronchi, whereas the stereoisomer D-NOARG was without effect. This inhibition was reversed by L-arginine (1 mM) but not be D-arginine (1 mM). The nitric oxide donor compound, 3-morpholinosydnonimine (SIN-1), was equipotent at relaxing the central and peripheral airways. Vasoactive intestinal peptide (VIP), although it relaxed central airways, was virtually ineffective in relaxing the peripheral airways. In addition, the peptidase, alpha-chymotrypsin, at a concentration that blocked relaxations to VIP, was without effect on NANC relaxations in the central bronchi. The results support the following hypotheses: (1) both central and peripheral airways receive nonadrenergic relaxant innervation; (2) the relaxant response to electrical stimulation of this system is dependent on a pathway involving L-arginine; and (3) the relaxant response does not appear to involve VIP, but it may involve the production of nitric oxide.