Results of a study of 59 patients with duodenal ulcer indicate that the blockader of serotoninergic receptors peritol (P) increases approximately two times the rheological properties of gastric mucus. In patients with uncomplicated form of the disease and basal hypersecretion P inhibits averagely by 1/3 the production of acid and pepsin. At the same time patients with hemorrhagic complications in the anamnesis show a marked increase of acid and pepsin secretion and a reduction of the aggregation activity of thrombocytes induced by adrenalin.