The cerebrospinal fluid pressure at the foramen of Monro in man in the recumbent position is less than 100 mm water relative to atmospheric pressure. The oscillations in the pressure wave due to respiration and cardiac pulsation vary with the actual pressure and increase as the overall pressure rises. In man lying horizontally the oscillation at the foramen of Monro is usually less than 50 mm water pressure, of which the cardiac component is about 15 mm and the respiratory component 35 mm water pressure. The fluid pressure within the cranial cavity is not uniform. In the recumbent face upwards position the pressure at the frontal pole is close to atmospheric or slightly subatmospheric but at the occipital pole is of the order of 160 to 190 mm water pressure. Examples are given showing the effect of posture on cerebrospinal fluid pressures in man and in the goat. The concentration of arachnoid granulations and venous lacunae near the vertex and the pressures in this region are discussed. The need for more precise methods of pressure measurement in the superior sagittal sinus is outlined by citing the Pitot tube. Pressure studies on patients with presenile dementia and dilated cerebral ventricles are reported.