Hydrocephalus was induced in adult rabbits by injection of silicone oil into the cisterna magna. The frontal horns of the lateral ventricles enlarged to the greatest degree and reached maximum size within 3 days. Cilia and microvilli were progressively lost from the ependymal surface primarily over the corpus callosum. Stretching and flattening of ependymal cells was most pronounced over the septal area. Mitotic activity was significantly increased among ependymal cells and subependymal astrocytes over the caudate nucleus and corpus callosum during the 2 weeks immediately following induction of hydrocephalus. Cerebrospinal fluid shunting returned the ventricles to normal size. Following shunting, however, the periventricular tissue did not resume a normal appearance. Longer durations of hydrocephalus were associated with less recovery of the periventricular tissue following shunting.