The oral administration of loxtidine to mice at doses of 600, 250, and 50 mg/kg/day for 746 days produced carcinoid tumours of the gastric fundus. The fundic mucosa also showed marked atypical hyperplasia with changes in foveolar cells similar to those seen in early incomplete metaplasia. These effects may be related to the prolonged achlorhydria produced by this potent unsurmountable histamine H2 receptor antagonist.