Three types of endocrine cells (G cells producing gastrin-17, D cells producing somatostatin, and GER cells containing endorphine) in the mucous membrane of the stomach antrum from 14 patients with duodenal ulcer and 10 healthy persons were studied. Biopsies were fixed in a modified Bowen solution and imbedded into paraffin. The slides were stained by Grimelins' method and immunohistochemically with the use of the peroxidase-antiperoxidase method. The number of cells per 1 mm2 of the mucous membrane was counted. Patients with ulcer have shown the increased number of G and GER cells and decreased number of D cells. Besides, pronounced G cell hyperplasia with a relative decrease of GER cells and a marked decrease of Grimelins-positive cells (as compared to other patients with duodenal ulcer) were observed in 3 out of 14 ulcer patients. The authors conclude that the alteration of the balance between antagonistic hormone effects results in the hypersecretory syndrome that plays the main role in the pathogenesis of duodenal ulcer.