We studied ultrastructural features of epithelial cells of the inner medullary collecting tubules in Brattleboro and Wistar under the action of desmopressin (dDAVP, 5 μg/100 g body weight for 2 days). Intracellular reorganization of transepithelial barrier for osmotic water transport depended on the capacity of rats to the synthesis of endogenous vasopressin.