The objective of the present study was to examine the role of the angiotensin II type 1 receptor (AT(1)-R) in the diabetes-aggravated oxidative stress and brain injury observed in a rat model of combined diabetes and focal cerebral ischemia. Diabetes was induced by an injection of streptozotoxin (STZ; 55 mg/kg iv) at 8 wk of age. Two weeks after the induction of diabetes, some animals received continuous subcutaneous infusion of the AT(1)-R antagonist candesartan (0.5 mg.kg(-1).day(-1)) for 14 days. Focal cerebral ischemia, induced by middle cerebral artery occlusion/reperfusion (MCAO), was conducted at 4 wk after STZ injection. Male Sprague-Dawley rats (n = 189) were divided into five groups: normal control, diabetes, MCAO, diabetes + MCAO, and diabetes + MCAO + candesartan. The major observations were that 1) MCAO produced typical cerebral infarction and neurological deficits at 24 h that were accompanied by elevation of NAD(P)H oxidase gp91(phox) and p22(phox) mRNAs, and lipid hydroperoxide production in the ipsilateral hemisphere; 2) diabetes enhanced NAD(P)H oxidase gp91(phox) and p22(phox) mRNA expression, potentiated lipid peroxidation, aggravated neurological deficits, and enlarged cerebral infarction; and 3) candesartan reduced the expression of gp91(phox) and p22(phox), decreased lipid peroxidation, lessened cerebral infarction, and improved the neurological outcome. We conclude that diabetes exaggerates the oxidative stress, NAD(P)H oxidase induction, and brain injury induced by focal cerebral ischemia. The diabetes-aggravated brain injury involves AT(1)-Rs. We have shown for the first time that candesartan reduces brain injury in a combined model of diabetes and cerebral ischemia.