According to the present investigations prostaglandins showed a specific morphologic effect on (rat) gastric mucosa. Namely: During their effect the foveolae of the mucosa have opened; - which is a convincing sign of mucus production. This phenomenon (i.e. the foveola-opening,) showed a direct correlation with the cytoprotective (anti-ulcerogenic) nic) effect of the investigated prostaglandins in the order of PG-I-2 > PG-E(1) > PG-F-2 alpha > 6-keto-PG-F-1 alpha.