The location and discovery of histidine decarboxylase and aromatic L-amino acid decarboxylase in the stomach are described. Feeding and gastrin-like agents stimulate increased gastric histidine decarboxylase (HD) acttvity in rats. Procedures which result in increased gastrin release - often by raising antral pH - also have this action in intact but not in antrectomised rats. Evidence for a histaminic feedback mechanism controlling HD levels is discussed. HD activity is reduced by protein synthesis inhibitors and by chronic pyridoxine deficiency. The effects of inhibitors of HD on gastric secretion in rat, dog and man are reviewed. The role of HD and histamine in gastric secretion is considered.