The angio-architectural and histological changes of small cerebral blood vessels in experimental hydrocephalus were studied to assess the changes of the vascular bed in the cerebral mantle. Change of the microvasculature assessed from microcorrosion casts by scanning electron microscopy (SEM) and the histological changes shown by light and electron microscopy were compared before and after shunting for hydrocephalus. The changes of the rCBF were also evaluated by the hydrogen clearance method. In hydrocephalus, a reduction in the number and caliber of the capillaries was noted in both the white and gray matter in the SEM study, but the capillaries were preserved and changes were mild and nonspecific in the electron microscopic examination. Shunting resulted in the reversal of all these changes to normal along with recovery of the rCBF, which had decreased in hydrocephalus. These observations suggest that changes of the vascular bed participate in the alteration of cerebral mantle width in the hydrocephalic process, and that the changes of the microvasculature result not only from damage to the capillaries themselves but also from changes of the perivascular structures.