Na-H exchanger NHE3 and Cl-anion exchanger CFEX (SLC26A6, PAT1) play principal roles in the reabsorption of Na and Cl in the proximal tubule of the mammalian kidney. The mechanisms by which NHE3 and CFEX are localized to and maintained in the brush border of the proximal tubule are largely unknown. To investigate the possible interaction of NHE3 and CFEX with the PDZ-domain-containing scaffolding protein PDZK1, we performed a series of in vitro interaction assays with GST-fusion proteins and native brush border membrane proteins. These studies demonstrated that, not only were NHE3 and CFEX capable of directly interacting with PDZK1, but that this interaction was mediated through their C-terminal PDZ-interaction sites. To determine whether PDZK1 interaction is essential for brush border localization of NHE3 and CFEX in vivo, we examined the expression of NHE3 and CFEX in kidneys of wild-type and PDZK1-null mutant mice by both Western analysis and immunocytochemistry. These studies indicated that, although brush border expression of NHE3 was unaffected by the loss of PDZK1, the expression of CFEX was markedly reduced. Finally, we assayed CFEX functional activity as Cl-oxalate exchange in brush border membrane vesicles and oxalate-stimulated volume absorption in microperfused proximal tubules. Consistent with the observed decrease in CFEX protein expression, both measures of CFEX functional activity were dramatically reduced in PDZK1-null animals. In conclusion, the scaffolding protein PDZK1 is essential for the normal expression and function of Cl-anion exchanger CFEX in the proximal tubule of the mammalian kidney.