In order to develop an experimental rodent model, we administered 2 ml (10(8) organisms/ml) broth culture of any of four human isolated strains of Helicobacter pylori by the oral route on a one-time basis to both BALB/c nude and BALB/c euthymic mice. After 20-wk examination, we have successfully demonstrated that the gastric mucosa of nude mice was continuously, and that of euthymic mice, temporarily (for 2 wk), colonized by orally administered, freshly isolated strains obtained from humans with gastritis, gastric ulcer, and duodenal ulcer, but never by the established strain. After colonization, gastritis and duodenitis were produced, and the presence of bacteria in gastric mucus was pathologically proved in all infected animals. We have confirmed that such colonization is always established when a large dose of a freshly isolated strain of H. pylori is administered at one time. We believe that this is the first reported rodent model of H. pylori-associated gastritis.