Helicobacter pylori is a major pathogenic factor in gastritis, ulcer disease and gastric cancer. Helicobacter pylori associated gastritis is of complex pathogenesis which is only partially elucidated. In the present study we investigated by immunohistochemical BSA method the expression of inducible nitric oxide synthase in the gastric wall and in vitro in Helicobacter pylori. In the gastric wall nitric oxide synthase immunoreactivity was found in macrophages, in endothelial cells and in neural elements. In Helicobacter pylori nitric oxide synthase was also expressed. In conclusion, we postulate a possibility of a direct toxic effect of Helicobacter pylori to gastric epithelial cells through the nitric oxide radical.