In addition to numerous basolateral K+-channels that recycle K + imported by the Na+-K +ATPase, parietal cells are thought to possess an apical K+-channel that is active only during acid secretion. The apical K+-channel could facilitate movement of cytoplasmic K ÷ into the gastric lumen and this K* would then be recycled by the H*-K*-ATPase during acid secretion. Here we show that the stomach possesses 2 inwardly rectifying K+-channels (Kir) that localize to the apical surface of parietal cells in the rat stomach. Methods: Nested primers were designed using rat brain Kit4.1 (GenBank X86818) and Kit5.1 (GenBank AF249676) nucleotide sequences, and RT-PCR was performed using rat stomach RNA. Purified PCR products cloned into a pCR 2.1-TOPO TA cloning vector were sequenced and subjected to BLAST analysis to verify that the amplification products corresponded to Kir channels. Frozen sections from the resting rat stomach were stained with anti-rat brain Kit4.1 antibody (kindly provided by Drs. Graham Wilkin and Chris Knott, Gila 30: 362, 2000) and anti-H+-K+-ATPase antibody (kindly provided by Dr. Adam Smolka). Results: KIN.1 and Kit5.1 mRNAs are expressed at low levels in the rat stomach. Antibody staining shows that Kir4.1 localizes at two sites. First, some cells in the neck region stain strongly with anti-Kir4.1 antibody, perhaps representing the presence of Kir4.1 in a type of stem cell precursor. Second, anti-Kir4.1 antibody localizes to the apical surface of parietal cells but does not co-localize with the H+-K+-ATPase. Instead, the signal is isolated to a domain that forms a thin continuous band along the apical surface. Conclusions: These finding show that Kir4.1 and KirS.1 mRNAs are expressed by rat stomach and that Kir4.1 (protein) is localized to the apical surface of parietal cells. Our results suggest that parietal cells have heteromeric Kir channels consisting of Kir4.1/Kir5.1. We postulate that Kir4.1/Kir5.1 moves cytoplasmic K + to the lumen during acid secretion and that this heteromeric channel is important for the regulation of gastric acid secretion.