The anti-secretory and anti-ulcer effects of prostaglandin E 2 (PGE 2) using iso-osmotic buffer as a vehicle have been investigated in several types of laboratory animals. Orally administered PGE 2 was found to be highly effective in preventing formation of ulcers in several experimental models -- pylorus ligated induced ulcers in rats, histamine induced ulcers in guinea pigs, reserpine induced ulcers in rats and pentagastrin induced ulcers in guinea pigs and cats. PGE 2 also suppressed acid secretion but not pepsin activity. It was concluded that the anti-ulcer effects of PGE 2 were due to its anti-secretory activity rather than antipepsin activity. In view of PGE 2's activity in preventing ulceration induced by histamine and reserpine in addition to pentagastrin, it is suggested that the anti-pentagastrin activity of PGE 2 is not specific.