The effects of smoking one cigarette on a plateau of submaximal gastric secretion induced by histamine acid phosphate (16 nmol kg-1 h-1 (0.005 mg kg-1 h-1)) were measured in eight normal volunteers and in eight duodenal ulcer subjects. The rate of secretion in both groups fell by 25% without any statistically significant change in pyloric loss or duodenogastric reflux. It is concluded that smoking reduces gastric secretion. The possibility that relative antral hypoacidity induced by regular smoking produces an increase in parietal cell mass via a feedback mechanism mediated by gastrin is discussed: such a mechanism would explain the previously reported positive correlation between maximal gastric secretion of acid and the total dose of cigarettes.