Inwardly rectifying K^+ channel, Kir4.1/KAB-2/Kir1.2, is expressed in glial cells of brain and distal tubular epithelia of kidney and is considered to participate in K^+ transport. We found that some subset of gastric mucosal cells of rat stomach expressed Kir4.1 mRNA by RT-PCR and in situ hybridization. By immunohistochemical analysis, Kir4.1 was found to be expressed in parietal cells. Immunoelectron microscopic examination further revealed that Kir4.1 was expressed in tubulovesicles and apical membrane of parietal cells and co-localized with H^+ /K^+ -ATPase. This suggests that Kir4.1 participates in acid secretion. Kir4.1 may supply K^+ ions to the K^+ site of H^+ /K^+ -ATPase to maintain the ATPase activity. SAP97/dlg, which contains three PDZ domains, was also found to be expressed on tubulovesicles and apical membrane of parietal cells. SAP97 could aggregate Kir4.1 on the membrane of transfected cells with SAP97 and Kir4.1. SAP97 may participate in the subcellular localization of Kir4.1 in parietal cells.