1. H+/K(+)-ATPases are members of the P-type ATPase multigene family. The prototypical H+/K(+)-ATPase is the protein that acidifies gastric luminal contents. The physiological and pharmacological significance of this pump has led to a detailed investigation of its biochemistry and molecular and cell biology. 2. Recently, a number of closely related H+/K(+)-ATPase isoforms have been discovered. These isoforms are present in organs other than the stomach, including the colon and kidney, where they contribute to acid-base and potassium homeostasis. The structure, expression and physiological roles of the gastric H+/K(+)-ATPase and other isoforms are reviewed.