15-Methyl PGE2 and 16,16-dimethyl PGE2 were found (1) to be 40 and 100 times, respectively, more potent than PGE2 after intravenous administration in inhibiting histamine-stimulated gastric secretion in dogs with a denervated (Heidenhain) gastric pouch, (2) to be active orally and intrajejunally, whereas PGE2 was inactive, and (3) to exert antisecretory activity for longer duration than PGE2. 16,16-Dimethyl PGE2 was about 2.5 times more potent than 15-methyl PGE2. Volume, acid concentration, and output, and pepsin output (but not concentration) were reduced in a dose-dependent manner. In the rat, 16,16-dimethyl PGE2 also inhibited gastric secretion and prevented the formation of ulcers produced by various methods: gastric ulcers (Shay, and steroid induced) and duodenal ulcers (secretogogue induced). In this species, 1l816-dimethyl PGE2 was 2 to 50 times more potent than PGE2, depending on the endpoint, and was active orally. These prostaglandins appear to inhibit gastric acid secretion by acting directly on the parietal cells, and making these unresponsive to most stimulants. Vomiting was a side effect of the prostaglandin analogues in the dog, but almost exclusively when these were given orally. After intravenous or intrajejunal administration at doses inhibiting gastric secretion by 80%, vomiting was seen only once. These results suggest that 15-methyl PGE2 and 16,16-dimethyl PGE2 may be of value in the treatment of peptic ulcer.