Duplicate pairs of isoforms of each of the NKCC2 and the NCC absorptive cation-chloride-cotransporters have been isolated from the European eel. As with mammalian NKCC2, NKCC2alpha isoform mRNA expression was restricted to renal tissues, whereas NKCC2beta isoform expression was present in intestine and urinary bladder. Similar to mammalian NCC, NCCalpha mRNA expression was also found in the kidney, whereas, expression of NCCbeta mRNA was found at low levels in a number of tissues but particularly in intestine. Following 3 weeks of transfer of yellow or silver (adult life stages) eels from freshwater (FW) to seawater (SW), renal mRNA expression of NKCC2alpha did not change whereas NCCalpha expression was reduced although only significantly in silver eels. This suggests that any changes in renal sodium chloride re-absorption in SW-acclimated fish may be due to decreased NCCalpha cotransporter activity rather than the result of suppression of NKCCalpha cotransporter activity. Intestinal mRNA expression of NKCC2beta generally increased following SW acclimation, although maximal increases occurred later in yellow (7 days) than silver (2 days) eels. Average levels of NKCC2beta mRNA abundance in the middle intestine were 89% of those in the anterior, and this was reduced to 44% (of the level in the anterior intestine) in posterior intestine/rectum. Expression of NCCbeta was only found in the posterior intestine/rectum. Together these results suggest intestinal sodium chloride absorption may switch from occurring via NKCCbeta to NCCbeta as imbibed fluid travels down the intestine and the concentration of luminal potassium decreases.