1. This study used whole-cell patch clamp and RNA in situ hybridization experiments to determine whether the cAMP-activated C1- current expressed in choroid plexus epithelial cells was carried by the cystic fibrosis transmembrane conductance regulator (CFTR) channel. 2. In patch clamp experiments, inclusion of 0.25 mM cAMP and 375 protein kinase A catalytic subunit (PKA) in the electrode solution caused activation of an inwardly rectifying current (21/23 cells). This current was C1- selective, since the current reversal potential (Erev) was -31 +/- 3 mV with equilibrium potential values for C1- (EC1) and Na+ (ENa) of -44 and 0 mV, respectively. 3. In anion substitution experiments, the relative anion permeability sequence for the inward rectifier was: I- (3.5) > HCO3-(1.5) = C1-(1.0) > Br-(0.6) > aspartate (0.2). 4. The inward rectifier was sensitive to inhibition by a range of known channel inhibitors, including: glibenclamide (100 microns), DIDS (100 and 500 microns), NPPB (100 microns) and Ba2+ (1 mM). 5. In RNA in situ hybridization experiments, using two independent rat CFTR cRNA probes, expression of CFTR could not be detected in epithelial cells from the rat choroid plexus. 6. In conclusion, the cAMP-dependent whole-cell C1- current present in choroid plexus epithelial cells from the rat has properties which are distinctly different from those of CFTR.