The effects of L-NG-nitro arginine (L-NOARG) on a-chymotrypsin-resistant, non-adrenergic, noncholinergic (NANC) relaxations of guinea-pig tracheal smooth muscle have been examined. L-NOARG (1-100pM), but not D-NOARG (100pM), inhibited the NANC relaxations in a concentration-related manner. The effects of L-NOARG were partially reversed by L-arginine but not D-arginine. L-NOARG was without effect on acetylcholine-induced contractile responses of the trachea or on relaxations produced by vasoactive intestinal peptide, sodium nitroprusside or isoprenaline. These results suggest that an endogenous nitrate may contribute to NANC relaxations of tracheal smooth muscle.Introduction In many species, including man and guinea-pig, electrically induced relaxations of tracheal smooth muscle have a non-adrenergic, non-cholinergic (NANC) component (Barnes, 1986). The identity of the neurotransmitter which mediates this effect is unknown and may vary with the species studied. In the guinea-pig, vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) are strong candidates for at least part of this NANC relaxant response. Ellis &