cAMP is a universal second messenger regulating a plethora of processes in the kidney. Two downstream effectors of cAMP are PKA and exchange protein directly activated by cAMP (Epac), which, unlike PKA, is often linked to elevation of [Ca 2+] i. While both Epac isoforms (Epac1 and Epac2) are expressed along the nephron, their relevance in the kidney remains obscure. We combined ratiometric calcium imaging with quantitative immunoblotting, immunofluorescent confocal microscopy, and balance studies in mice lacking Epac1 or Epac2 to determine the role of Epac in renal water-solute handling. Epac1−/− and Epac2−/− mice developed polyuria despite elevated arginine vasopressin levels. We did not detect major deficiencies in arginine vasopressin [Ca 2+] i signaling in split-opened collecting ducts or decreases in aquaporin water channel type 2 levels. Instead, sodium–hydrogen exchanger type 3 levels in the proximal tubule were dramatically reduced in Epac1−/− and Epac2−/− mice. Water deprivation revealed persisting polyuria, impaired urinary concentration ability, and augmented urinary excretion of Na+ and urea in both mutant mice. In summary, we report a nonredundant contribution of Epac isoforms to renal function. Deletion of Epac1 and Epac2 decreases sodium–hydrogen exchanger type 3 expression in the proximal tubule, leading to polyuria and osmotic diuresis.—Cherezova, A., Tomilin, V., Buncha, V., Zaika, O., Ortiz, PA, Mei, F., Cheng, X., Mamenko, M., Pochynyuk, O. Urinary concentrating defect in mice lacking Epac1 or Epac2.