To evaluate the efficacy of angiotensin II receptor blockers (ARBs) for use in the treatment of diabetic nephropathy, we examined the effects of olmesartan medoxomil (olmesartan), an angiotensin II type 1 (AT1) specific ARB, on the progression of nephropathy in Zucker diabetic fatty (ZDF) rats, an animal model of type 2 diabetes. We used 2 doses of olmesartan, a sub-antihypertensive dose and an antihypertensive dose, to specifically examine whether the drug exerts beneficial effects on the kidney without lowering blood pressure. Olmesartan mixed in the diet at a concentration of 0.001% (approximately 0.6 mg⁄kg⁄day) or 0.01% (approximately 6 mg⁄kg⁄day) was administered for 19 weeks starting from 12 weeks of age, when the animals developed microalbuminuria. Lean non-diabetic rats served as controls. ZDF rats had hyperglycemia, hyperinsulinemia, and moderate hypertension as compared to lean control rats. Plasma glucose and insulin concentrations were not affected by olmesartan, and blood pressure was lowered only by the high dose of olmesartan. Progressive proteinuria in ZDF rats was greatly (about 70%) suppressed by the high dose of olmesartan and moderately (about 30%) suppressed by the low dose that did not significantly lower blood pressure. ZDF rats exhibited hyperlipidemia and hypoalbuminemia, both of which were substantially corrected by treatment with olmesartan. The histological evidence of glomerular and tubular damage in the ZDF rats was also reduced by the drug. These results indicate that AT1 receptor blockade with olmesartan retards the progression of nephropathy associated with type 2 diabetes without affecting glucose metabolism, and that this renal protective effect is at least partly independent of the antihypertensive effect of the drug. (Hypertens Res 2002; 25: 271-278)