This is a double-blind, randomized, cross-over study of men and women (21-64 years old) with type 1 or type 2 diabetes mellitus and albuminuria (³30 mg/g creatinine). Participants will be randomized to a MR antagonist + placebo or HCTZ + potassium supplementation for 6 weeks. The MR antagonist arm will receive eplerenone 50 mg daily. The HCTZ arm will receive HCTZ 12.5 mg with potassium 10 Meq daily. Amlodipine 5 to 10 mg daily will be added during run phase to control blood pressure. Blood pressure goal is less than 130/80 mm Hg. There will be a 4-week washout period before the patients are crossed-over to the other study arm. MRI perfusion reserve, brachial artery reactivity, and blood samples will be obtained at the beginning and end of each treatment arm.