In the search for gastric ATPases that might be related to the mechanism of HCl secretion, an interesting and rather unique K+-stimulated ATPase has been discovered. This enzyme is isolated from oxyntic cells and has been associated with the apical plasma membrane and/or tubulovesicular system. Membrane vesicles containing the K+-stimulated ATPase transport H+ into the vesicular lumen under the appropriate conditions of ATP, Mg2+, and KCl. This process can be measured by pH electrode, binding of certain metachromatic dyes to "energized" sites, or accumulation ratios of substances with appropriate pK values. Vesicular interior can be acidified to pH 3.5 or below. At the present time, it is difficult to distinguish between an electrogenic H+ pump and an electroneutral H+/K+ exchange mechanism. A hypothetical scheme for the gastric H+ secretory mechanism is proposed which fits much of the data from studies on the K+-ATPase, vesicular transport, and intact gastric mucosa.