The aim of these studies was to determine to what extend Na⁺-coupled bicarbonate transporters participate in transepithelial movement of base equivalents. The epithelial distribution of transporters was investigated by RT-PCR, immunoblotting, and immunohistochemistry. Function was estimated from intracellular pH measurements by fluorometry using pH sensitive dyes. Duodenum secretes HCO₃^- at high rates and absorbs CO₂. Both NBCn1 and NBCe1 are expressed in the basolateral membrane. Low stilbene sensitivity and studies on NBCn1 knockout mice strongly suggests NBCn1 to be the main base loader of these cells. U. Seidlers group has shown that basal and stimulated duodenal HCO₃^- output is diminished in NBCn1 knockout mice. NBCn1 is also expressed basolaterally in renal medullary thick ascending limbs (mTAL) and collecting ducts (CD). In mTAL, NBCn1 seems stimulated by increased luminal NH₄⁺ delivery and the protein is suggested to supply cells with HCO₃^- to buffer H⁺ formed from NH₄⁺. Therefore, NBCn1 was suggested to participate in the medullary NH₄⁺ short-cut, while the protein may simply maintain cellular pH in CD. Choroid plexus secretes cerebrospinal fluid and possibly controls its pH as well. Two baseloaders are expressed in the basolateral membrane. The stilbene sensitivity and studies on slc4a10 knockout mice indicate that this protein is the most important base loader in this epithelium, whereas NBCn1 is sometimes observed luminally. NBCe2 is expressed in the luminal membrane where P.Brown group has shown that it mediates outward transport. In conclusion, Na⁺-coupled bicarbonate transporters may participate in transepithelial base movement in epithelia with high HCO₃^- transport rates.