Several peptides are important regulators of gastric acid secretion. The best characterized is gastrin. This circulating hormone is produced in the gastric antrum and mediates the gastric phase of acid secretion. Somatostatin is important as an inhibitory regulator of acid secretion, but the relative importance of paracrine versus endocrine delivery to its targets remains to be determined. The mammalian bombesin peptides, GRP27 and GRP10, probably are mediators of neural release of gastrin. Opioid peptides in the gastric wall appear to act as endogenous neurostimulants of gastric acid secretion under some conditions. Other neuropeptides in the gastric mucosa and submucosa, including sensory neuropeptides, may be important regulators of acid secretion. Hormones or nerves activated by fat and other substances in the lumen of the small intestine cause inhibition of acid secretion, but the specific peptides responsible for this effect have not yet been identified from a long list of candidates. The effects of peptides on the parietal cell and the gastrin and somatostatin cells are functionally linked with those of cholinergic and adrenergic nerves and with locally released histamine.