Obstructive hydrocephlus was produced in 10-14 day-old rabbits by injection of kaolin into the cisterna magna and the ependyma and subependymal tissue was studied by electron microscopy. Generally, the study confirmed recent light microscopic observations on similar models (Torvik et al., 1976). In contrast to most previous reports, it was found that the ependyma adapted remarkably well to ventricular dilatation. No true ependymal defects occurred even in extensive hydrocephalus except at the sites of the ventricular synechiae which sometimes ruptured. The specialized ependymal junctions remained intact but outside the junctions the intercellular clefts were widened, particularly along the lateral wall of the lateral ventricle. The density of the microvilli and cilia decreased, probably because of the increase in the surface area of the ependyma. Dense bundles of filaments developed in the ependymal cells of the hydrocephalic animals. The extracellular space of the subependymal white matter appeared increased but there was no evidence of destruction of fibres or cells. Thus, the reduction of the cerebral mantle thickness was probably mainly caused by pre-sure atrophy.