The authors studied acid and nonparietal secretion and the level of the blood flow in the gastric mucosa combined with the determination in it of the concentrations of prostaglandins (PGE and PGF2 alpha) and cyclic nucleotides (cAMP and cGMP) in 39 patients with primary (mostly medio-gastric) gastric ulcers, in 22 patients with secondary gastric lesions (with relation to duodenal ulcer) and in 58 patients with peptic ulcer. 30 healthy persons were investigated for control. More active acid secretion of the stomach was shown in the patients with secondary ulcers (as compared to the patients with primary ulcers) with similar indices of bicarbonate and fucose. The deficit of PGE in gastric mucosa was observed in ulcers of different sites with its lowest values in secondary gastric ulcers. In the patients with primary and secondary gastric ulcers the level of mucosal PGF2 alpha was decreased, in duodenal ulcer it was normal. A higher rate of the gastric blood flow was observed in the patients with duodenal and secondary gastric ulcers, and a normal one-in primary gastric ulcers. Normal gastrinemia was noted in ulcers of different sites. The development of secondary gastric ulcers in patients with duodenal ulcers appeared to result from insufficiency of prostaglandin biosynthesis in the gastric mucosa.