Endothelin (ET) is a powerful vasopressor agent that is activated in a number of pathophysiologic states where renal perfusion is reduced. Since renal vasoconstriction occurs as part of renal aging, we investigated the possibility that ET may be activated in the old kidney. These experiments involved acutely blocking endogenous ET with Bosentan (a non-peptide mixed antagonist to both ET receptor types ETA and ETB), in Sprague-Dawley male rats of various ages: young (4 5 months), middle-aged (12-13 months) and old (19-20 months). Experiments were performed in chronically catheterized, conscious rats, studied under unstressed conditions. Renal hemodynamics and sodium excretion were measured before and during acute ET receptor blockade. In all three age groups, Bosentan had no effect on glomerular filtration rate (GFR), renal plasma flow (RPF), renal vascular resistance (RVR), blood pressure (BP) or urine flow. Sodium excretion increased significantly with Bosentan but the natriuresis was similar in rats of all ages. These results suggest that ET does not contribute to the renal vasoconstriction of the old rat kidney.