The authors used the scanning electron microscope to study the surface ultrastructure of the ependyma in six control rabbits and six rabbits subjected to bilateral jugular vein ligation four to five days previously. Whereas in the control rabbits the mean CSF pressure was 27 mm H2O, it rose to 71.5 mm H2O in the experimental animals. The ependymal lining of the fourth ventricle, third ventricle and the inferior horn of the lateral ventricle was examined. In the experimental animals, clusters of cilia emanating from ependyma were separated. This appeared secondary to ingrowth of cell processes covered with microvilli. Remarkable alterations were seen in the dorsal and infundibular regions of the third ventricle. While a marked reduction in ciliary density and appearance of pits were noticeable in the former, almost total replacement of globular excrescences by pleomorphic microvilli was noted in the infundibular region. The morphological alterations in the ependyma that occurred in experimental animals suggested a modification in its function from a surface capable of propelling cerebrospinal fluid by ciliary movement to one capable of increased ependymoabsorption. Increased incidence of macrophage-like supraependymal cells was noted in the experimental rabbits.