Nitric-oxide-synthase inhibitors and lipopolysaccharides (LPS) were tested on guinea-pig tracheas. Each trachea sample was mounted to a perfusion holder and connected to a differential pressure transducer via side hole catheters. Concentration response curves were generated by adding increasing, cumulative amounts of the various inhibitors to the tracheal baths. The inhibitors N-omega-nitro-L- arginine-methyl-ester (L-NAME), NG-methyl-L-arginine (L-NMMA), and aminoguanidine were tested on trachea already responding to methacholine or histamine. These inhibitors did not affect the pressure differences across the trachea, nor did they increase contractile responses to methacholine or histamine. These facts suggested that guinea-pig airways were not influenced by nitric- oxide (10102439) (NO). However, NO production was up regulated by the addition of LPS, observed in the decreased tracheal reactivity to both methacholine and histamine in the presence of LPS. Also, L- NAME increased reactivity to methacholine with LPS present in the tracheal baths. LPS only affected responses to histamine when the epithelium was present, suggesting that histamine response was related to the epithelium and methacholine response, since it occurred with or without the epithelium present, was related to smooth muscle. The authors conclude that lipopolysaccharides induce nitric-oxide-synthase, since addition of L-NAME in the presence of LPS potentiates contractions caused by methacholine. Without LPS present, NO does not appear to have a major role in regulating guinea-pig airway reactivity.