Ion transporters play a central role in gastric acid secretion. To determine whether some of these transporters are necessary for the normal ultrastructure of secretory membranes in gastric parietal cells, mice lacking transporters for H+, K+, Cl-, and Na+ were examined for alterations in volume density (Vd) of basolateral, apical, tubulovesicular and canalicular membranes, microvillar dimensions, membrane flexibility, and ultrastructure. In mice lacking Na+/H+ exchanger 1 (NHE1) or the Na+-K+-2Cl- cotransporter (NKCC1), the ultrastructure and Vd of secretory membranes and the secretory canalicular to tubulovesicular membrane ratio (SC/TV), a morphological correlate of secretory activity, were similar to those of wild-type mice. In mice lacking Na+/H+ exchanger 2 (NHE2) or gastric H+, K+ -ATPase alpha- or beta-subunits, the SC/TV ratio and Vd of secretory membranes were decreased, though canaliculi were often dilated. In H+, K+ -ATPase-deficient parietal cells, canalicular folds were decreased, normally abundant tubulovesicles were replaced with a few rigid round vesicles, and microvilli were sparse, stiff and short, in contrast to the long and flexible microvilli in wild-type cells. In addition, microvilli of the H+, K+ -ATPase-deficient parietal cells had centrally bundled F-actin filaments, unlike the microvilli of wild-type cells, in which actin filaments were peripherally positioned concentric to the plasmalemma. Data showed that the absence of H+, K+ -ATPase produced fundamental changes in parietal cell membrane ultrastructure, suggesting that the pump provides an essential link between the membranes and F-actin, critical to the gross architecture and suppleness of the secretory membranes.