Studies were designed to examine the effect of a selective endothelinA (ETA) receptor antagonist, BQ123, on severe postischemic acute renal failure (ARF) in Sprague-Dawley rats. Severe ARF was induced in uninephectomized, chronically instrumented rats by 45-min renal artery occlusion. BQ123 (0.1 mg/kg.min) or vehicle was infused intravenously for 3 h on the day after ischemia. Measurements before infusion (24 h control) showed a 98% decrease in glomerular filtration rate (GFR), increase in fractional excretion of sodium from 0.6 to 39%, and in plasma K+ from 4.3 to 6.5 mEq/liter. All vehicle-treated rats died in 4 d because of continuous deterioration of renal function, resulting in an increase of plasma K+ to fatal levels (> 8 mEq/liter). Infusion of BQ123 significantly improved survival rate (75%) by markedly improving tubular reabsorption of Na+ and moderately increasing GFR and K+ excretion. Plasma K+ returned to basal levels by the 5th d after ischemia. Improved tubular function was followed by gradual recovery in GFR and urinary concentrating mechanism. Additional data from renal clearance studies in rats with moderate ARF (30-min ischemia) and in normal rats with intact kidneys showed that ETA receptor blockade increases Na+ reabsorption and has no effect on renal hemodynamics. These results indicate that in the rat, the ETA receptor subtype mediates tubular epithelial function, and it plays a significant role in the pathogenesis of ischemia-induced ARF. Treatment with the selective ETA receptor antagonist reverses deteriorating tubular function in established ARF, an effect of possible therapeutic significance.