4. Patients with a ventriculoperitoneal (VP) or ventriculoatrial (VA) shunt must have an on/off device in their shunt systems to be eligible for the study. Patients must be able to tolerate shunt closure for >/= 4 hours without development of clinical signs of increased intracranial pressure. Patients unable to tolerate shunt closure for >/= 4 hours will not be eligible for the study.