Cystic fibrosis is characterized by an impaired cyclic adenosine 3,5-monophosphate (cAMP) activated Cl- conductance in parallel with an enhanced amiloride sensitive Na+ conductance (ENaC) of the respiratory epithelium. Very recently, acute downregulation of ENaC by the cystic fibrosis transmembrane conductance regulator (CFTR) was demonstrated in several studies. The mechanism, however, by which CFTR exerts its inhibitory effect on ENaC remains obscure. We demonstrate that cytosolic domains of human CFTR are sufficient to induce inhibition of rat epithelial Na+ currents (rENaC) when coexpressed in Xenopus oocytes and stimulated with 3-isobutyl-1-methylxanthine (IBMX). Moreover, mutations of CFTR, which occur in cystic fibrosis, abolish CFTR-dependent downregulation of rENaC. Yeast two hybrid analysis of CFTR domains and rENaC subunits suggest direct interaction between the proteins. Enhanced Na+ transport as found in the airways of cystic fibrosis patients is probably due to a lack of CFTR dependent downregulation of ENaC.