The parietal cells, which are responsible for the production of gastric HCl acid, are uniquely equipped for high-gradient ion transport. Adequate energy is supplied by oxidative metabolism in the mitochondria, which occupy an exceptionally high proportion of the cytoplasmic volume. Another characteristic feature is the secretory canaliculi. These are tortuous small channels lined by microvilli which penetrate all parts of the cytoplasm and which expand during stimulation of secretion. The activity of the parietal cell is controlled by receptors for acetylcholine, histamine and gastrin on the basolateral cell membrane. Stimulation of these receptors modulates the levels of protein kinases in the cell and brings about the changes from resting to stimulated structure. A key role in the production of acid is played by the gastric acid pump, also known as the H+, K(+)-ATPase, which exports hydrogen ions in 1:1 exchange for potassium ions. This protein is a member of the P-type ATP-driven ion pumps and appears to be uniquely located in the parietal cell. The gastric acid pump is found in the tubulovesicular membranes of the resting cell and moves to the membrane lining the secretory canaliculus when acid secretion is stimulated. Functional acid secretion also requires the presence of KCl pathways in the secretory membrane in order to supply the acid pump with a source of potassium ions. For each hydrogen ion secreted across the secretory membrane, one bicarbonate ion is generated in the cytoplasm and is transported across the basolateral membrane in exchange for chloride. The movement of ions across the apical membrane is followed osmotically by water, resulting in the secretion of 160 mM HCl from the parietal cell.