The effect of inhibiting the renin-angiotensin system was evaluated in male Sprague-Dawley rats with reduced renal mass produced by right nephrectomy and infarction of two-thirds of the left kidney. Separate groups of rats were then administered the angiotensin receptor antagonists, A-81988 or losartan (DuP 753), the angiotensin converting enzyme inhibitor, enalapril, or vehicle (tap water) in their drinking water for 4 weeks. Tail cuff blood pressures and blood samples were obtained weekly. Excretory function during week 4 was evaluated using metabolic cages. Rats with reduced renal mass were characterized by a significant elevation in systolic blood pressure and urinary protein excretion along with a reduced urine osmolality. At 1 mg/kg/day, A-81988 prevented the hypertension and the development of proteinuria. A-81988 administration also improved urinary concentrating ability because urine osmolality was significantly higher in this group compared to untreated controls. The same dose of losartan or enalapril was ineffective at controlling the development of the hypertension, indicating that A-81988 is more potent in vivo. Despite the maintenance of systemic hypertension, losartan significantly blunted the proteinuria compared to vehicle-treated controls. At a dose of 10 mg/kg/day, losartan and enalapril also prevented the increase in systolic blood pressure and proteinuria and produced an increase in urine osmolality. These data support the hypothesis that angiotensin receptor antagonists have beneficial effects in forms of renal failure associated with proteinuria and diminished concentrating ability.