Praomys (Mastomys) natalensis were sensitised by peritoneal injection of ovalbumin mixed with aluminium hydroxide. The animals developed homocytotropic IgE and IgG 1 antibodies. When presensitised animals were challenged with ovalbumin in the gastric wall, a gastric ulcer appeared. This ulceration, induced by an immunological release of histamine, could be inhibited by the administration of disodium cromoglycate. The present model is useful for producing a gastric ulcer in a determined site. It supports the theory of a possible allergic component in the pathogenesis of the human disease.