Abstract.Many intracellular organelles contain a unique primary, electrogenic proton pump termed the vacuolar H+‐ATPase. This pump, found in many endocytic, secretory, and storage vesicles in fungal, plant and animal cells, functions, in conjunction with a chloride conductance, to acidify the vesicle interior. Although remotely related to the mitochondrial ATP synthase, the vacuolar H+‐ATPase is a distinct pump which differs in inhibitor sensitivity, subunit composition and function. The vacuolar H+‐ATPase transports only protons, and permeable anions (chloride) are required for optimal vesicle acidification. Allosteric and regulatory effects are not yet fully understood. Vesicle acidification appears to be essential for receptor‐mediated endocytosis, protein synthesis, and secretion and storage of small solutes such as neurotransmitters. A similar plasma membrane‐located H+‐ATPase may contribute to urinary acidification and cell pH