The vascular apparatus in congenitally hydrocephalic rat brains was studied. The characteristic findings were structural changes in the small vessels with formation of an intracerebral cavity in the periventricular white matter. Stenotic or occluded blood vessels were located in an edematous area adjacent to the dilated lateral ventricles. Electron microscopically, the endothelial cells of capillaries and venules in the periventricular edematous region showed an irregular surface with indentations, and contained numerous plasmalemmal vesicles. Occluded capillaries were found in the border zone between the edematous and nonedematous area. These vascular changes indicate the presence of a disturbed microcirculation in the hydrocephalic brain.