A study was made of the effect of a synthetic enkephalin analog and cimetidinee on experimental duodenal ulcer in rats induced by cysteamine administration. The ulcers were demonstrated to heal within 4 weeks. It was noted that ulcer occurrence was preceded by the increased serum gastrin level. Administration to rats of the enkephalin analog accelerated ulcer healing to a greater extent than that of cimetidin. It is suggested that one of the mechanisms of enkephalin analog protective action might involve the prevention of the gastrin increased level.