Hydrochloric acid is involved in the causation of peptic ulcer, but the exact role has not been defined. Suppression of acid secretion is associated with ulcer healing. The acid secreting cell is the parietal cell, which possesses a proton pump in the secretory membrane; morphologic changes accompany and facilitate the active secretion of hydrochloric acid. Stimulation of acid secretion occurs by three major pathways, which utilize acetylcholine, histamine, and gastrin. The predominant effects of histamine are mediated by adenylate cyclase, whereas those of gastrin and acetylcholine involve cytosolic calcium. There is a complicated arrangement of receptors and pathways that culminate in the activation of the proton pump. The parietal cell is influenced by neurocrine, hormonal, and paracrine mechanisms. Peptides join the more familiar neurotransmitters in affecting the parietal cell. Somatostatin is present in the gut and acts to decrease acid secretion. The hormone gastrin is released, in a feedback fashion, when the antrum is alkalinized. Most stimuli of acid secretion are blocked by H2-antagonists. Inhibitory hormones are released when acid arrives in the intestine. Inhibition of acid secretion can be achieved by influencing the parietal cell at the level of histamine, gastrin, and muscarinic receptors. The proton pump itself can be blocked by drugs that inhibit the final phase of acid secretion.