Intracranial pressures (cisternal cerebrospinal fluid and brain surface pressures) were measured in nine anesthetized dogs during and after experimental hypercapnic hypoxia. Both pressures increased significantly after 20 minutes of hypoventilation with the hypoxic mixture (PaO220 ± 7.3 torr; PaCO287 ± 7.9 torr). When the dogs were subsequently hyperventilated with oxygen, the cerebrospinal fluid pressure rapidly declined, but pressure at the brain surface remained twice the control value. This pressure dissociation was even more striking when the dogs were then given an infusion of distilled water intraveously. These findings suggest that swelling of the brain occurs during hypercapnic hypoxia and is not reversed by an hour of hyperventilation. The lack of correlation between cisternal CSF and brain surface pressures suggests that CSF was displaced from the cranium while brain volume expanded. CSF pressure did not, therefore, reflect the actual pressure of the brain.