Acute intramedullary infusion of hyperosmotic NaCl, used to simulate a high-salt diet-induced increase of medullary osmolality, increases urine production and endothelin release from the kidney. To determine whether endothelin mediates this diuretic and natriuretic response, urine flow and Na(+) excretion rate were measured during acute intramedullary infusion of hyperosmotic NaCl in anesthetized rats, with or without endothelin receptor antagonism. Isosmotic NaCl was infused into the left renal medulla during an equilibration period and 30-min baseline period, followed by hyperosmotic NaCl for two additional 30-min periods. Hyperosmotic NaCl infusion significantly increased urine flow of vehicle-treated rats (from 5.9 ± 0.9 to 11.1 ± 1.8 μl/min). Systemic ET(B) receptor blockade enhanced this effect (A-192621; from 7.7 ± 1.1 to 18.7 ± 2.9 μl/min; P < 0.05), ET(A) receptor blockade (ABT-627) had no significant effect alone, but the diuresis was markedly attenuated by combined ABT-627 and A-192621 administration (from 4.4 ± 0.7 to 5.4 ± 0.9 μl/min). Mean arterial pressures overall were not significantly different between groups. Surprisingly, the natriuretic response to hyperosmotic NaCl infusion was not significantly altered by systemic endothelin receptor blockade, and furthermore, intramedullary ET(B) receptor blockade enhanced the diuretic and natriuretic response to hyperosmotic NaCl infusion. ET(A) receptor blockade significantly attenuated both the diuretic and natriuretic responses to hyperosmotic NaCl infusion in ET(B) receptor-deficient sl/sl rats. These results demonstrate an important role of endothelin in mediating diuretic responses to intramedullary infusion of hyperosmotic NaCl. Moreover, these data suggest ET(A) and ET(B) receptors are both required for the full diuretic and natriuretic actions of endothelin.