Introduction: We have previously demonstrated the increased salt sensitivity of blood pressure (BP) in diabetic patients with early nephropathy. Here, we examined the effects of an angiotensin II receptor blocker (ARB) on salt sensitivity and renal oxidative stress or nitric oxide (NO) in those patients. Patients and methods: Type 2 diabetic patients with ( n = 6) and without ( n = 6) microalbuminuria were studied on a high-salt diet for one week and on a salt-restricted diet for one week. The study was repeated in the patients with microalbuminuria during treatment with an ARB, valsartan (80 mg/day). Salt sensitivity was assessed from the BP/sodium excretion curve. Urinary excretion rates of NOx, 8-hydroxy-2-deoxyguanosine as a marker of oxidative stress, and plasma tetrahydrobiopterin as a cofactor for NO synthase were measured. Results: Compared with diabetic patients without microalbuminuria, patients with microalbuminuria showed greater salt sensitivity and lower urinary excretion of NOx. In the patients with microalbuminuria, treatment with valsartan reduced salt sensitivity in association with increased NOx excretion, reduced 8-hydroxy-2,-deoxyguanosine excretion, and increased plasma tetrahydrobiopterin levels. Conclusions: These data support the hypothesis that ARBs reduce the salt sensitivity of BP by decreasing renal oxidative stress and restoring NO activity in diabetic patients with microalbuminuria.