Adenosine deaminase activity was studied in endoscopically taken slices from gastric mucosa in patient after partial gastric resection performed due to complicated duodenal ulcer, and currently with peptic ulcer in the stump. The samples of gastric mucosa were taken before and after 6 weeks of treatment with ranitidine, 150 mg twice daily, at a distance within 2 cm and greater than 2 cm from the ulcer crater. Adenosine deaminase activity was measured in mucosa homogenates by determination of ammonia liberated from substrate. It was found that therapy with ranitidine was accompanied by an increase in enzyme activity in the mucosa surrounding unhealed stump ulcers, while no changes were noted in mucosa around healed stump ulcers. A possible role of mucosal adenosine deaminase activity in stump ulcer healing is postulated.