A role for nitric oxide in the H3-histaminergic agonist-induced inhibition of the non-adrenergic, non-cholinergic (NANC) contraction has been studied in guinea-pig perfused bronchioles. (R)-alpha-Methylhistamine ((R)-alpha-MeHA), an agonist for H3 receptors, inhibited the NANC contraction induced by electrical field stimulation. NG-Nitro-L-arginine methyl ester (L-NAME) (50 microM), an inhibitor of nitric oxide synthesis, blocked the effect of (R)-alpha-MeHA. The effect of L-NAME was reversed by L-arginine (50 microM). L-NAME, L-arginine or (R)-alpha-MeHA were without effect on exogenous substance P- or neurokinin A-induced contractile responses of the perfused bronchioles. These results show that an H3-agonist inhibited the release of neurotransmitters in NANC nerve endings of guinea-pig perfused bronchioles presumably by production of nitric oxide.