Gastric acid secretion, gastric emptying, fasting serum gastrin and the serum gastrin response to a meal were measured in duodenal ulcer patients before, and at least five days after completing prolonged treatment with cimetidine (1 or 2 g/day for four or eight weeks followed by 600 mg twice daily for six months). Fasting serum gastrin and the gastrin response were also measured during treatment. Compared with pretreatment values, fasting serum gastrin concentrations were raised both during and five to 31 days after stopping treatment (P less than or equal to 0.02). The integrated gastrin response and the rate of gastric emptying of the solid component of the meal were increased during treatment (P less than 0.001 and P less than 0.002 respectively) but returned to pretreatment levels after stopping therapy. No significant changes were observed in the basal or maximal acid outputs or the rate of emptying of the liquid component of the meal. The results imply that the hypergastrinaemia associated with long-term cimetidine therapy does not result in increased gastric acid secretion.