The results from the treatment of 25 patients with duodenal ulcer with 1 g tagamet daily (3x200 mg after meal and 400 mg in the evening before going to bed) in the course of 20 days, are reported. The subjective complaints of the patients disappeared in 87--100 per cent (in 2/3 of the patients even in the first three days after the initiation of the treatment) and the ulcer in 44 per cent. The immediate results obtained are statistically more significant as compared with the control group (only on diet), but the differences are not significant as regards the groups treated only with oxyferoscorbon and only with Caved-S. Tagamet treatment leads to a decrease of the basic indices of gastric secretion and does not change the content of N-acetylneuroamine acid in the gastric juice. The venous administration of tagamet inhibits the gastric acid production stimulated by pentagastrin. Tagamet treatment does not change the biochemical laboratory indices, the inflammatory and structural changes in the gastric-duodenal mucosa and induces no adverse effects and complications.