That the cerebral vessels respond by dilatation to the inhalation of amyl nitrite has already been noted by various investigators. Schuller,1 directly observing the surface of the brain through the intact dura, noted that amyl nitrite caused (in rabbits) dilatation of all cerebral vessels. Mosso2 measured the volume of the brain and the pulse through a defect in the skull in three patients. He concluded that since amyl nitrite always caused an increase in the bulk of the brain, the drug must have caused dilatation of cerebral vessels. Hurthle3 drew conclusions concerning the diameter of the cerebral vessels from measurements of blood pressure in the circle of Willis in rabbits and cats. He inferred from his observations that amyl nitrite caused cerebral dilatation. Biedl and Reiner4 employed a similar method, measuring also venous output. They, too, concluded that amyl nitrite caused cerebral vasodilatation. Shepard,5 using