The expression pattern of the electroneutral Na⁺,HCO₃^- cotransporter NBCn1 (slc4a7) was investigated by -galactosidase staining of mice with a LacZ insertion into the NBCn1 gene. This method is of particular value as it is independent of immunoreactivity. We find that the NBCn1 promoter is active in a number of tissues where NBCn1 has previously been functionally or immunohistochemically identified; including a broad range of blood vessels (vascular smooth muscle cells and endothelial cells), kidney thick ascending limb and medullary collecting duct epithelial cells, the epithelial lining of the kidney pelvis, duodenal enterocytes, choroid plexus epithelial cells, hippocampus and retina. Kidney corpuscles, colonic mucosa and non-vascular smooth muscle cells (from the urinary bladder, trachea, gastrointestinal wall and uterus) were novel areas of promoter activity. Atrial but not ventricular cardiomyocytes were stained. In the brain, distinct layers of the cerebral cortex and cerebellar purkinje cells were stained as was the dentate nucleus. No staining of skeletal muscle or cortical collecting ducts was observed. RT-PCR analyses confirmed the expression of NBCn1 and -galactosidase in selected tissues. Disruption of the NBCn1 gene resulted in reduced NBCn1 expression; and in bladder smooth muscle cells reduced amiloride-insensitive Na⁺-dependent HCO₃^- influx was observed. Furthermore, disruption of the NBCn1 gene resulted in a lower intracellular steady-state pH of bladder smooth muscle cells in the presence of CO₂/HCO₃^- but not in its nominal absence. We conclude that NBCn1 has a broad expression profile, supporting previous findings based on immunoreactivity, and suggest several new tissues where NBCn1 may be important.