The transport stoichiometry of the electrogenic sodium-bicarbonate cotransporter (SLC4A5 or NBCe2) in mouse choroid plexus was examined. Whole-cell recording methods measured the currents carried by the NBCe2, using experimental solutions determined to minimise the contributions of the other ion conductances present. Increases in outward current were observed when 21.2 mM HCO3(-) was added to the bath solution in the presence of Na(+), but not N-methyl-D-glucamine. This HCO3(-)-induced current was completely abolished by 500 microM 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid. The reversal potential for the HCO3(-)-induced current was -95.1+/-7.1 mV (n=11), a value which corresponds to a NBCe2 transport stoichiometry of 3 HCO3(-) with 1 Na(+). The NBCe2, with this stoichiometry, will mediate the efflux of HCO3(-) and Na(+) from the cell into the cerebrospinal fluid at the apical membrane of the choroid plexus.