The influence of the rate of ventricular enlargement on the morphology of hydrocephalic white matter was studied and correlated with previous studies of water content. Different rates of ventricular enlargement were obtained in two groups of cats by opening either the calvaria or the calvaria and the dura mater before injecting kaolin into the cisterna magna. Animals from each group underwent in vivo fixation of brain 2, 3, and 6 weeks after hydrocephalus was induced. Specimens of white matter were taken 1, 2, and 3 mm lateral to the ependymal surface of the lateral ventricles, imbedded, and examined using transmission electron microscopy. The ultrastructural changes associated with ventricular enlargement varied with the model used and the duration of hydrocephalus. Marked expansion of the extracellular space extending 2 mm lateral from the ependyma was found in the craniectomy-durectomy preparations examined 2 to 3 weeks after kaolin injection. Time-matched craniectomy preparations had less enlargement of the extracellular space that was confined to the white matter immediately adjacent to the ventricle. Marked glial reaction was observed in these areas in the early craniectomy preparations. When studied 6 weeks after hydrocephalus induction, both models had less expansion of the extracellular spaces compared to early observations. Glial reaction was found in both models, but was greater in the craniectomy model. The correlation of these morphological findings with the rate of ventricular enlargement and earlier studies of water content are discussed.