In some patients with increased intracranial pressure there are irregular rapid fluctuations of the pressure. They can be found in the course of routine lumbar puncture in quiet (or sedated) patients by observing the pressure for a few minutes. These pressure changes have been mentioned, briefly and without explanation, in reports of cases of severe head injury,* brain tumor,† cerebral pseudotumor,‡ and acute cerebral vascular disease.6The purpose of the present study is to seek the origin and significance of these fluctuations. THE CEREBROSPINAL DURA AS AN ONCOMETER Under pressures compatible with life, the cerebrospinal dura is essentially not distensible.7It contains the central nervous system, cerebrospinal fluid (CSF), and blood, all of which are practically incompressible. The CSF pressure is the sum of the partial tensions exerted by neural tissue, CSF, and intradural blood. The sudden rises and falls in pressure which are the subject of this