The gastric acid response to a 200-ml cup of tea was measured by in situ titration in 36 patients with duodenal ulcer (DU) and 56 without duodenal ulcer (controls). Tea resulted in an acid secretory response which was almost equal to that after a maximal dose (0.04 mg/kg) of histamine. The effect of tea was mainly due to its local chemical action on gastric mucosa. Tea without milk and sugar resulted in an acid response higher than that evoked by a maximal dose of histamine. The concentration of tea brew that had the greatest effect on gastric acid secretion was 15 g/200 ml, which was three times as much as that in a palatable cup of tea. Tea is a potent stimulant of gastric acid, and this can be reduced by adding milk and sugar.