We studied the effect of NO synthesis blockade on the hydrouretic function and the state of renal interstitial matrix in WAG rats with normal blood vasopressin level and Brattleboro rats lacking endogenous vasopressin. In WAG rats, impaired osmotic concentration accompanied by an increase in the content of histochemically detectable glycosaminoglycans in the inner medullary interstitium was observed under conditions of NO synthesis blockade. In Brattleboro rats, NO synthesis inhibition, on the contrary, induced an antidiuretic response without significant changes in the renal extracellular matrix. These findings attested to a significant influence of NO on the urine concentration process. Morphofunctional changes in the kidneys under conditions of NO synthesis blockade are determined by blood vasopressin level.