Blockade of the renin-angiotensin system reduces cardiovascular morbidity and mortality in diabetic patients. Angiotensin II (Ang II) plays an important role in the structural and functional abnormalities of the diabetic heart. We investigated whether or not Ang II type 1 receptor blocker (ARB) could attenuate left ventricular (LV) remodeling in male mice with diabetes mellitus (DM) induced by the injection of streptozotocin (200 mg/kg, i.p.). Diabetic mice were treated with candesartan (1 mg/kg/day; DM+Candesartan, n=7) or vehicle (DM+Vehicle, n=7) for 8 weeks. Heart rate and aortic blood pressure were comparable between the groups. Normal systolic function was preserved in diabetic mice. In contrast, diastolic function was impaired in DM+Vehicle and was improved in DM+Candesartan, as assessed by the deceleration time of the peak velocity of transmitral diastolic flow (40.3+/-0.3 vs. 37.3+/-0.5 ms, p<0.01) and the time needed for relaxation of 50% maximal LV pressure to baseline value (tau; 10.6+/-0.7 vs. 8.7+/-0.6 ms, p<0.05) without significant changes in heart rate and aortic blood pressure. Improvement of LV diastolic function was accompanied by the attenuation of myocyte hypertrophy, interstitial fibrosis and apoptosis in association with the expression of connective tissue growth factor (CTGF) and myocardial oxidative stress. Moreover, candesartan directly inhibited Ang II-mediated induction of CTGF in cultured cardiac fibroblasts. ARB might be beneficial to prevent cardiac abnormalities in DM.