It has been proposed that autosomal dominant polycystic kidney disease (ADPKD)affected renal epithelial cells undergo a phenotypic transition from a highly differentiated absorptive state to a much less differentiated secretory state during cystogenesis and that this transition is accompanied by loss of epithelial cell polarity and mistargeting of specific membrane proteins. We conducted a detailed evaluation of this hypothesis in the Pkd2^WS25/- mouse model of ADPKD. Ultrastructural analysis of Pkd2^WS25/- cysts by electron microscopy confirmed that cystic epithelial cells progressively dedifferentiate with cyst enlargement. Immunocytochemical analysis of both early- and late-stage cysts with antibodies directed against Na⁺-K⁺-ATPase, Ksp-cadherin, and E-cadherin failed to detect evidence of altered cyst cell polarity. Na⁺-K⁺-ATPase and Ksp-cadherin were expressed exclusively on the basolateral membranes (BLM) of epithelial cells in all early cysts. Expression levels of both Na⁺-K⁺-ATPase and Ksp-cadherin decreased progressively with the degree of cyst cell dedifferentiation, but neither protein was ever mislocalized. Highly dedifferentiated cysts did not express immunodetectable levels of either Na⁺-K⁺-ATPase or Ksp-cadherin. E-cadherin was expressed prominently on the BLM of all cysts. Cysts were subsequently stained with an antibody directed against the secretory isoform of the Na⁺-K⁺-Cl^- cotransporter NKCC1. NKCC1 expression was detected on the BLM of advanced cysts only. Our data are consistent with a model of progressive cystic epithelial cell dedifferentiation in which fluid accumulation in late-stage cysts is mediated by transepithelial secretion of chloride rather than secretion of sodium by apical Na⁺-K⁺-ATPase.