In duodenal ulceration the Alcian blue staining acid mucosubstances in goblet cells are reduced and the goblet cells themselves disappear. At the same time periodic acid-Schiff (PAS) staining neutral mucosubstances appear in the cells of the surface epithelium. These changes have been measured quantitatively with the help of a Vickers M86/0010 Scanning and Integrated Microdensitometer. During healing the changes are reversed. It is suggested that these changes suggest a metaplasia towards a gastric type mucosa as a protective response to the presence of ulceration.