Guinea-pig trachea possesses an inhibitory (bronchodilator) non-adrenergic, non-cholinergic (i-NANC) innervation associated with cholinergic nerves, which may be mediated, at least in part, by nitric oxide (NO). L-NG-Nitro-arginine methyl ester (L-NAME), an inhibitor of endogenous NO synthesis, enhanced cholinergic responses to electrical field stimulation with no effect on contractile responses to exogenous acetylcholine or excitatory NANC (e-NANC) bronchoconstriction. The effects of L-NAME were reversed by L-arginine but not D-arginine. These results suggest that NO released by nerve stimulation modulates cholinergic neurotransmission in guinea-pig trachea.