The direct involvement of the cystic fibrosis transmembrane conductance regulator (CFTR) in the epithelial secretion of HCO₃^– is contentious (1, 3). However, there are a limited number of epithelia that, like the guinea pig pancreatic duct (2), secrete isosmotic NaHCO₃. The ileum of the marsupial, Trichosurus vulpecula, appears to have a pure HCO₃^– secretory response, as cAMP-dependent secretagogues stimulate a Cl^– independent, HCO₃^– dependent short circuit current, which is inhibited by serosal 4,4’-diisothiocyano-stilbene-2,2’-disulfonate (DIDS), but is insensitive to serosal bumetanide. In view of this we have investigated whether CFTR plays a role in HCO₃^– secretion by this tissue. To do this we have used RT-PCR and in situ hybridization studies, plus Western blots, to investigate the expression of CFTR in the possum ileum. These studies were combined with Ussing chamber studies of the intact tissues and characterization of the properties of the cloned possum CFTR (pCFTR) expressed in Fischer rat thyroid (FRT) cells, using both transepithelial measurements of chloride current through the apical membrane of confluent sheets of FRT cells and whole cell patch clamp measurements of single FRT cells. Transcript for pCFTR was expressed in the ileal tissue of the possum and this was located predominantly in the base of the villi, with declining levels of expression along the length of the villi. Western blots of protein extracted from ileal epithelial cells confirmed that mature CFTR was expressed in the ileal epithelial cells. Consistent with these observations the HCO₃^– secretory response of the ileum was inhibited by mucosal (m) 5 nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) (K_i≈29µM), but not DIDS (1mM, m) or glibenclamide (250µM, m). A similar pharmacology was seen for cloned pCFTR when expressed in FRT cells; NPPB inhibiting pCFTR (K_i=3.7µM), whereas glibenclamide had little effect (K_i≈500 µM) and DIDS (1 mM) had no effect. Despite the comparable pharmacology between the cloned channel and the native tissue, pCFTR had a relatively low permeability to HCO₃^– (P_HCO3😛_Cl=0.22±0.04., n=5). The expression of CFTR in the possum ileum combined with the comparable pharmacology of the HCO₃^– secretory response of the intact tissue and the cloned pCFTR, implicate CFTR in the secretion of HCO₃^– by this tissue. However, the relatively low permeability of pCFTR to HCO₃^– questions the role of CFTR in HCO₃^– secretion, unless it is expressed at high levels in the HCO₃^– secretory cells.