Diabetes mellitus is characterized by hyperglycemia and is well established to associate with oxidative stress. The present study examines whether impaired myogenic response of middle cerebral artery MCA and autoregulation of cerebral blood flow (CBF) in our T2DN diabetic rats is due to hyperglycemia and oxidative stress induced actin cytoskeleton reorganization, and if this contributes to dementia. We identified that both young (4-month) and older (18-month) T2DN diabetic rats exhibit impaired pressure-induced myogenic response in isolated MCAs. Forced dilatation occurred at pressures above 140 mmHg in MCAs isolated from elderly T2DN rats with mild hypertension but not controls. Cortical blood flow measured by laser Doppler flowmetry rose by 137 ± 15% and 36 ± 5%, respectively, in T2DN and SD rats when MAP was increased from 100 to 180 mmHg. Autoregulation of CBF was shifted to lower pressures in elderly T2DN rats and they exhibited breakthrough at pressures above 140 mmHg. The F-Actin distribution area was significantly reduced in the vascular smooth muscle cells (VSMCs) freshly isolated from MCAs of T2DN rats compared with normal Sprague Dawley (SD) rats. The F-actin was disrupted similarly in VSMCs treated with high glucose or H 2 O 2 . Superoxide production is elevated in fresh isolated middle cerebral arteries (MCA) in T2DN diabetic rats compared to normal SD rats. We also found increased production of superoxide in primary VSMCs isolated from normal SD rats cultured in high glucose condition or freshly isolated from diabetic rats. T2DN rats exhibited neurons death in the hippocampus and cortex. Elderly T2DN rats showed memory disabilities. These results indicate that hyperglycemia induced elevated superoxide production causes actin cytoskeleton reorganization in cerebral VSMCs of diabetic T2DN rats, which may contribute to impaired cerebral vascular function, neurodegeneration and cognitive impairment.