['Exhaled nitric oxide is increased in asthma, but the mechanisms controlling its production, including the effects of T-helper type 2 (Th2) cytokines, are poorly understood. In mouse and submerged human epithelial cells, Th2 cytokines inhibit expression of inducible nitric oxide synthase (iNOS). Arginases have been proposed to contribute to asthma pathogenesis by limiting the arginine substrate available to NOS enzymes, but expression of any of these enzymes has not been extensively studied in primary human cells.', 'We hypothesized that primary human airway epithelial cells in air-liquid interface (ALI) culture would increase iNOS expression and activity in response to IL-13, while decreasing arginase expression.', 'iNOS and arginase mRNA (real-time PCR) and protein expression (Western blot and immunofluorescence) as well as iNOS activity (nitrite levels) were measured in ALI epithelial cells cultured from bronchial brushings of normal and asthmatic subjects following IL-13 stimulation.', 'IL-13 up-regulated iNOS mRNA primarily at a transcriptional level in epithelial cells. iNOS protein and activity also increased, arginase1 protein expression decreased while arginase 2 expression did not change. The changes in iNOS protein correlated strongly with changes in nitrites, and inclusion of arginase (1 or 2) did not substantially change the relationship. Interestingly, iNOS mRNA and protein were not correlated.'] Are iL-13 induced increases in nitrite levels primarily driven by increases in inducible nitric oxide synthase as compared with effects on arginases in human primary bronchial epithelial cells?