The paper presents the methodology of conducting those investigations designed to study the intestinal phase of gastric secretion, with the duodenal pH having been taken as the integral index. An acetylcholine test has been developed suggesting to us a preservation of the inhibitory duodenogastral reflex. The study into the gastric secretion in 69 patients with complications of peptic ulcer showed that the intraduodenal pH was disturbed in 39.1 percent of the patients, which fact warrants medicamentous and surgical correction to be instituted in them.