Specific pathogen-free Mongolian gerbils were infected orally with Helicobacter pylori to establish a new small animal model of severe gastritis H. pylori was recovered by culture from both antrum and body over a 16-week period after a single inoculation. The number of H. pylori colonising the antrum was about 100-fold higher than in the body, and this was consistent throughout the experiment. Histological examination showed that all animals developed severe inflammation with infiltration of polymorphonuclear leucocytes and mononuclear cells into the lamina propria and submucosa of the antrum from 4 weeks after infection. From 8 weeks after infection, multifocal lymphoid follicles appeared in the lamina propria and submucosa, and micro-erosions were also observed in the epithelial layer. At 16 weeks after infection, ulceration with disruption of the lamina muscularis mucosae was observed in the antral mucosa. To determine whether H. pylori caused gastritis or not, infected gerbils were treated with amoxycillin. After the treatment, gastritis could not be seen in the gastric mucosa. Therefore, the Mongolian gerbil is a useful small animal model to study the pathogenesis of H. pylori in gastric ulceration and severe gastritis and to assess anti-H. pylori treatment.