The intracranial pressure "reserve" test seems to be the most reliable method of determining when the brain's natural mechanisms for pressure compensation for added intracranial volume have been compromised or exhausted. The test employs a timed sequence of intracranial fluid injections, but as a safety precaution injections are discontinued if intracranial pressure remains elevated more than 10 Torr over baseline. In this case, a linear extrapolation is then calculated to determine the elevation which might have been achieved by a full series of injections. However, this linear extrapolation has been criticised on the expectation that an exponential response should be expected. A series of experimental observations in dogs and baboons and a review of clinical records in humans have been made to determine the observed slope of increase following aliquot injection during performance of the intracranial pressure reserve test. In these species the observed response was actually linear in shape rather than exponential. This held true even for different initial baseline values and with different volumes of "lesion" balloon inflations in experimental animals. A theoretic explanation is proposed.