A noncholinergic, nonadrenergic nervous system has been described, involving the sensory nerves in the airways. Chemicals, dusts and other irritants stimulate these sensory nerves to release substance P and related neuropeptides. These neuropeptides have the remarkable ability to affect multiple cells in the airways and to provoke many responses including cough, mucus secretion, smooth muscle contraction, plasma extravasation and neutrophil adhesion. This series of effects is termed "neurogenic inflammation." An enzyme exists on the surfaces of all lung cells that contain receptors for these neuropeptides. This enzyme, neutral endopeptidase (NEP), by cleaving and thus inactivating the neuropeptides, limits the concentration of the neuropeptide that reaches the receptor on the cell surface. Thus, neurogenic inflammatory responses are normally mild and presumably protective in nature. However, when NEP is inhibited pharmacologically (with NEP inhibitors) or by cigarette smoke, respiratory viral infection, or by inhalation of the industrial pollutant toluene diisocyanate, neurogenic inflammatory responses are exaggerated. Delivery of exogenous human recombinant NEP inhibits neurogenic inflammation. Finally, evidence is provided that corticosteroids suppress neurogenic plasma extravasation and that this drug can upregulate NEP in human airway tissue. Neutral endopeptidase cleaves multiple peptides. Thus, its selectivity resides, at least in part, on its fixed location on the surfaces of specific cells where it can modulate effects of peptides exposed to the cells' surfaces.