To study the role of nitric oxide synthase (NOS) in rat experimental bronchial asthma. 3H-arginine/3H-L-citrulline conversion technique was used to assay NOS activity of rat lung tissue and histochemical staining method for detect NADPH-d diaphorase. The results revealed that there were significant increase in the level of iNOS activity in asthma group from 152.39% to 249.40%, but the cNOS activity reduced from 64.84% to 61.81% (P < 0.05-0.01). Histochemical staining of NADPH-d showed deep staining of trachial and bronchial epithelium in asthma group. These results suggested that NOS plays a role in regulating airway inflammation and bronchial responsiveness. cNOS possesses a down regulatory effect, while iNOS upregulatory. The occurrence of airway inflammation is earlier than that of smooth muscle contraction and endothelial injury.