The effect of graded change of the CSF amount on the clinical status, intracerebral pressure and brain bioelectrical activity was studied in 31 patients operated on for brain tumors of various sites. Normally, the graded change in the CSF amount made with an object of examining the biophysical characteristics of the intracranial system did not produce the impairment of the patients status. Discomfort sensation that occurred in rare cases was not a consequence of either hypo- or hypertension as such but of the absence of CNS ability to adapt rapidly to the changes of the pressure in the cranium. The graded change in the CSF amount and thus in the CSF pressure caused minimum transitory changes in the intracerebral pressure of interstitial liquid and in brain bioelectrical activity. The data obtained indicate the feasibility of studying the biophysic characteristics of the intracranial system by means of graded changes of the CSF amount in neurosurgical patients undergoing acute postoperative period.