The cerebral blood volume (CBV) is an important parameter of cerebral hemodynamics. However, measurement of CBV has been difficult. We have devised a photoelectric method for measurement of regional CBV. The sensor of the apparatus was made of three components; a microlamp, a photodiode to which an infra-red filter was attached, and a polyethylene balloon, or polyethylene catheter with its tip open for intracranial pressure (ICP) measurement. These were pasted with silicon rubber. The sensor was applied to the brain surface to measure the intensity of the optical absorption (OA) of hemoglobin in the brain tissue. OA remained nearly constant despite various alterations in cerebral blood flow (CBF), brain tissue pressure, and water content of the brain. Both OA and ICP increased during vasodilatation induced by CO(2) inhalation, compression of the jugular veins, or intravenous injection of Papaverine hydrochloride. Both decreased during vasoconstriction induced by hyperventilation or severe arterial hypotension. These results suggest that OA reflects changes of CBV. The disadvantages of this method are that the absolute CBV and the CBV in the deeper brain tissues can not be measured and that the operative field has to be kept dark. However, this method has several advantages; (1) Regional CBV is continuously measured without radionuclide or other specific indicators. (2) This method does not require a skilled technique. (3) The cerebral transit time can be measured by injection of saline or other indicators into the right brachial artery. (4) This device can also measure ICP.