&NA;During experimental supratentorial epidural compression of the brain, cerebrospinal fluid (CSF) pressure and pressure‐volume index (PVI) were measured in the supra‐ and infratentorial compartments of the craniospinal axis in anesthetized ventilated cats. A transtentorial gradient of CSF pressure developed when CSF pressure in the lateral ventricle exceeded 20 mm Hg at balloon volumes of 0.4 to 1.2 ml. The shapes of both the pressure vs. volume and the pressure gradient vs. volume curves were exponential. PVI in the supratentorial compartment was lowered more rapidly as brain compression advanced than PVI in the infratentorial compartment. An index was calculated to represent the relative resistance to the flow of CSF across the tentorial hiatus during supratentorial brain compression. This revealed different resistances to the flow of CSF in the cephalic and caudal directions. A hypothesis concerning the hydrodynamic aspects of the development of tentorial herniation is presented. (Neurosurgery19:1‐8, 1986)