The purpose of this study was to determine the microvascular characteristics that cause cerebral cortical blood flow autoregulation to shift to a higher range of arterial pressures during established hypertension in spontaneously hypertensive rats (SHR). An open-skull technique with constant suffusion of artificial cerebrospinal fluid (PO2= 40–45 mm Hg, pCO2= 40–45 mm Hg, pH = 7.35–7.45) was used to view the parietal cortex of 18- to 21-week-old SHR and Wistar Kyoto (WKY) normotensive control rats. The resting inner diameters of first (1A)-, second (2A)-, and fourth (4a)- order arterioles were significantly (p< 0.05) smaller, and the wall thickness/lumen diameter ratios were significantly (p< 0.05) larger in SHR compared to WKY. Only 1A and 4A had significantly (p< 0.05) increased vessel wall cross-sectional area in SHR. At the resting mean arterial pressures of WKY and SHR, the passive (10−4M adenosine, topical) diameters of comparable types of arterioles were not significantly different (p> 0.05). At reduced arterial pressures, however, the arterioles in SHR had smaller maximum diameters than in WKY. Cortical blood flow in WKY and SHR was constant at arterial pressures from 70–150 mm Hg and 100–200 nun Hg, respectively. Resting arteriolar pressures in 1A, 2A, and 3A of SHR were substantially and significantly (p< 0.05) elevated, although pressures in the smallest arterioles and venules of WKY and SHR were similar. Therefore, it is possible that cerebral capillary pressure is only slightly elevated, if at all, in SHR as a result of the vasoconstriction. The number of arterioles per unit area of brain surface at rest was equal in WKY and SHR. In addition, the number of vessels was equal in WKY and SHR during maximal dilation, and neither type of rat demonstrated an opening of previously closed vessels upon maximum dilation. Therefore, the cerebral arteriolar constriction in SHR, which was probably potentiated by vessel wall hypertrophy of the largest and smallest arterioles, was the major contributor to an upward shift in the autoregulatory range, the protection of exchange vasculature pressures, and the increase in vascular resistance.