Changes in the antral gastric mucosa of patients with doudenal ulcer after a long-term administration of antisecretory agents were studied. The subjects were five patients with a different duration of the disease. The methods applied included light and electron microscopy of tissue samplings taken from the antral part of the stomach during the periods of exacerbation and remission. In addition to well-known morphological changes, cellular principalis not typical of antral gastric mucosa were found in one patient during exacerbation and remission periods.