In 20 patients with expanding intracranial lesions the cerebral blood flow was determined by the method of Kety-Schmidt in an own modification with an original device--N2O-meter, with simultaneous continuous measurement of the intraventricular pressure and arterial blood pressure. This made possible determination of the cerebral perfusion pressure and of the effect of its fluctuations on the cerebral blood flow. Lowering of the cerebral blood flow below the normal value occurred when the perfusion pressure fell below 60 mm Hg, but its further fall, even to 20 mm Hg, caused no proportional decrease of the cerebral blood flow.