Previous studies have provided functional evidence for electrogenic sodium bicarbonate cotransport in hepatocytes and in intrahepatic bile duct cholangiocytes. The molecular identity of the transporter(s) mediating electrogenic sodium bicarbonate cotransport in the liver is currently unknown. Of the known electrogenic sodium bicarbonate cotransporters (NBC1 and NBC4), we have previously shown that NBC4 mRNA is highly expressed in the liver. In the present study we utilized RT-PCR, immunoblotting, and immunohistochemistry to characterize the expression pattern of NBC4 in rat liver and kidney. For immunodetection, a polyclonal antibody against rat NBC4 was generated and affinity purified. Of the known human NBC4 variants, only the rat NBC4c orthologue was detected by RT-PCR in rat liver and the molecular mass of the NBC4c protein was ~145 kDa on immunoblotting. On imunohistochemistry, NBC4c was detected in hepatocytes and in the cholangiocytes lining the intrahepatic bile ducts. In hepatocytes, NBC4c was localized to the basolateral plasma membrane whereas intrahepatic cholangiocytes stained apically. The NBC1 electrogenic sodium cotransporter variants kNBC1 and pNBC1 were not detected by immunoblotting and immunohistochemistry in rat liver. The pattern of localization of NBC4c in the liver suggests that the cotransporter plays a role in mediating sodium bicarbonate cotransport in hepatocytes and intrahepatic cholangiocytes. Unlike the liver, the rat kidney expressed both electrogenic sodium bicarbonate cotransporter proteins kNBC1 and NBC4c. In kidney, NBC4c had a molecular mass of ~145 kDa and was immunolocalized to uroepithelial cells lining the renal pelvis where the cotransporter may play an important role in protecting the renal medulla from alterations in urine pH.