Prostaglandins of the E series have been shown, both in animals and humans to produce gastrointestinal antisecretory and antiulcer effects. Enprostil, a modified allenic prostaglandin E was found to be a highly potent inhibitor of gastric HCl secretion in a variety of species. In rats, in which both the pylorus and esophagus were ligated, p.o. ED50 values and 95% CL for inhibiting acid secretion evoked by histamine, pentagastrin and carbachol were 9.9 (6.7-15), 40 (11-145) and 0.83 (0.78-0.89) micrograms/kg, respectively. In inhibiting histamine-evoked acid secretion, enprostil was more potent when administered p.o. than when injected into the duodenum or s.c. When enprostil was injected directly into the pouch of Heidenhein dogs, intense antisecretory activity occurred, ED50 = 0.9 (0.7-1.1) micrograms/kg, whereas, when given p.o. to the main stomach the ED50 was 6.6 (3.2-13.6) micrograms/kg. Administration of cimetidine either p.o. or to the pouch resulted in virtually identical ED50 values, viz., 2.9 and 3.1 mg/kg. Enprostil also inhibited dimaprit- and pentagastrin-induced acid secretion in cats with permanent gastric fistulae. The oral ED50 values for inhibiting acid secretion evoked by these two secretagogues were 2.5 (1.4-4.3) and 0.8 (0.5-1.5) micrograms/kg, respectively. Enprostil was extremely potent in preventing indomethacin plus "cold stress" ulcers in rats. When given orally the ED50 was 0.61 (0.31-1.22) and s.c. it was 22 (9.0-52) micrograms/kg. It was also highly potent in preventing cysteamine-induced duodenal ulcers when given p.o., ED50 = 20 (17-23) micrograms/kg. Thus, enprostil is a highly potent antisecretory and antiulcer agent. It appears to act topically; directly at gastric mucosal sites.