✓ Supratentorial epidural pressure (EDP) was recorded during posterior fossa surgery in 14 patients. It was found that the operative position had great influence on the supratentorial EDP and that a correct position, especially of the head, was important in the control of the intracranial pressure. Rapid and extensive ventricular drainage may set up a secondary rise in pressure and must be avoided. Craniectomy alone did not have any definite decompressive effect as judged from the EDP. Splitting of the dura or resection of a tumor usually resulted in a fall in the EDP in patients with previously increased pressure. Inadequate ventilation as well as hyperventilation was rapidly manifested through variations in the EDP. EDP monitoring combined with single determinations of PaCO2 was a valuable guide in securing adequate ventilation.