Abstract: Objective: To explore the role of nitric oxide(NO) and substance P(SP) in the pathological mechanism of allergic rhinitis. Methods: The concentration of serum NO in 40 patients with allergic rhinitis and 30 healthy controls was detected by the nitrate reductase method. The expressions of inducible nitric oxide synthase (iNOS) and SP in the inferior nasal concha mucosa were detected by the strept avidin-biotin complex (SABC) immunohistochemistry method.　Results: The level of serum NO in patients with allergic rhinitis was significantly higher than that in the controls (52.42±18.73μmol/L vs 28.67±10.35μmol/L, t=6.26, P＜0.01). A large number of iNOS positive cells, mainly expressed in mucosa epithelial, submucos glandular, endothelial and submucos inflammatory cells were found in the concha mucosa in allergic rhinitis patients; but few iNOS positive cells could be found in the controls (27.5±3.2 vs 4.3±1.7, per scope, t=36.03, P＜0.01). The number of SP positive cells was 23.5±4.6 in allergic rhinitis patients and 5.3±3.1 in the controls(t=18.7，P＜0.01）. There were positive correlations between the number of iNOS positive cells and of SP positive cells (r=0.69, P＜0.01). Conclusion: The results suggest that NO and SP may play an important role in the pathogenesis of allergic rhinitis. SP can promote the production of NO by iNOS, which induces the process of allergic inflammation.

Key words: Allergic rhinitis, Nitric oxide, Nitric oxide synthase, Substance P