Enzymatic and non-enzymatic production of nitric oxide in humans: role in inflammation and host defence. Recent studies indicate that nitric oxide (NO) may play an important role in first line of defense in the airways and the stomach, since bacteriostatic concentrations of this gas has been found in the lumen of these organs. Airway NO synthesis is mostly carried out by a high producing "inducible like" NO synthase constantly present in the epithelium of the paranasal sinuses. Stomach NO synthesis, on the other hand, is non-enzymatic and results from acidification of salivary derived nitrite. Excess NO production has been implicated in the pathogenesis of inflammation although the exact role of NO is still unclear. NO production is enhanced in the mucosa of inflammatory diseases such as asthma and ulcerative colitis. Measurements of local NO production may be done in an easy way in the airways and the gastrointestinal tract by simply analyzing the concentrations of NO gas in luminal air of these hollow organs. Such non-invasive methods may be useful not only to explore what role NO plays in inflammation and host defence but possibly also in the diagnosis and monitoring of inflammatory mucosal diseases in the airways and gastrointestinal tract.