Intraperitoneal injection of Semax (synthetic analogue of ACTH4-7, MEHFPGP) in a dose of 50 mg/kg produced a protective effect on rats with experimental indomethacin-induced ulcers. Experiments on narcotized rats showed that Semax in the studied dose had no effect on basal blood flow in the stomach, but prevented reduction of blood flow induced by indomethacin. The antiulcer effect of Semax is probably related to improvement of blood flow in the gastric wall disturbed by indomethacin.