Objective: To observe the distribution of inducible nitric oxide synthase (iNOS) in gingival tissues in elderly patients with periodontitis. Methods: Gingival tissue samples were obtained during tooth extraction.The localization of positive expression of iNOS in gingival tissues in 10 patients aged 65-86 years with periodontitis was tested with immunohistochemical method and was compared with that of 10 adult patients(37-54 years old) with periodontitis, 10 juvenile patients (18-29 years old) with periodontitis and 10 healthy elders(65-81 years old),respectively. Results:iNOS expression was mainly found in epithelial cells, inflammatory cells in connective tissues, granuloma tissues, epithelial basement membrane and microvascular endothelial cells in gingiva in the 3 groups of periodontitis. The cases of positive expression of iNOS in microvascular endothelial cells and inflammatory cells in connective tissues in elderly patients with periodontitis were significantly fewer than those in juvenile patients and adult patients ( P