Nitric oxide (NO) is produced by resident and inflammatory cells in the respiratory tract by the enzyme NO synthase (NOS), which exists in three isoforms: neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS. NO production is increased in patients with COPD, and the production of NO under oxidative stress conditions generates reactive nitrogen species that may amplify the inflammatory response in COPD.