Marine teleosts produce Ca precipitates in the intestine as a product of osmoregulation. Ca precipitates are formed by a chemical reaction of Mg2+ and Ca2+ derived from ingested seawater with bicarbonate (HCO3−). It has been reported that HCO3− originates from the intestine; however, the pancreas is predicted to be another organ that may supply HCO3− to the intestinal tract. In the present study, the pancreas was surgically removed from Japanese eel to confirm its contribution to Ca precipitate formation. Pancreatectomized eel produced significantly less Ca precipitates than control eel in seawater, indicating that HCO3− from the pancreas contributes substantially to the formation of Ca precipitates. To further examine the molecular mechanisms of HCO3− secretion, we cloned cDNAs encoding HCO3− transporters and identified those transporters that elevated their mRNA expression in the intestine and pancreas following seawater transfer. In the intestine, mRNA expression of Slc26a6A was increased shortly after seawater transfer, whereas Slc26a1 mRNA expression increased gradually following seawater transfer. In the pancreas, Slc26a3 mRNA expression was high during the early stage of seawater acclimation, whereas Slc26a1 expression increased gradually after transfer to seawater. In the intestine and pancreas, therefore, both transient and progressively increasing types of HCO3− transporters are likely to be involved in HCO3− secretion into the intestinal lumen in a coordinated manner. J. Exp. Zool. 319A:53–62, 2013. © 2012 Wiley Periodicals, Inc.