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Background and Objectives:Hyper-responsivenes to nonspecific stimuli is a characteristic finding of all allergic diseases. It the whole mechanism has not been clearly established. We aimed to understand the role of inducible nitric oxide synthase (iNOS) expresion in nasal hyper-responsivenes of rhinosinusitis. Materials and Method:1) To develop a platelet activating factor (PAF)-induced rhinosinusitis rat model, 50 μl of 16 μg/ml PAF was applied into the nose of rats. At days 1, 3 and 5, the rats were killed. 2) μl of 10 μM capsaicin was applied intranasaly and the amount of microvascular leakage in the nasal mucosa was measured with Evans blue asay at days 1, 3 and 5 in the rhinosinusitis model and the control rats. 3) To examine the efect of iNOS, 75 mg/kg of aminoguanidine was pretreated systemicaly 1 hour before the application of capsaicin. 4) To localize the expresion of iNOS, immunohistochemical staining was performed using the avidin-biotin-peroxidase method with an anti-iNOS antibody. Results:Induction of rhino-ium, varied according to the time interval, were observed. A significant enhancement of microvascular leakage was clearly demon-strated by topicaly applied capsaicin, which was completely blocked by aminoguanidine, the iNOS inhibitor in the PAF-induced rhinosinusitis. The expression of iNOS was localized in the inflamatory cells infiltrated in the mucosa. Conclusion:The expression of iNOS in the inflamatory cells as well as epithelial damage related to eosinophil infiltration may cause nasal hyper-responsivenes. (Korean J Otolaryngol 203 ;46 :35-41)