Nitric oxide synthase (NOS) is a cytosolic enzyme that produces nitric oxide (NO ). Evidence is accumulating that the constitutive form of NOS is present in various pneumocytes releasing NO at a low basal rate to control vascular tone and act as a modulator of cellular responses. In response to inflammatory cytokines various lung cells also exhibit the upregulation of an inducible NOS (iNOS) capable of generating NO at levels high enough to cause oxidant damage. This review summarizes evidence that endotoxin, an etiologic agent in organic dusts, can induce NO production following in vitro or in vivo exposure in a variety of lung cell types. Induction of NO is also demonstrated in response to inhalation of cotton dust in rats. Species variability is discussed.