Cerebral arterial pressure and relative changes in blood flow have been measured in contiguous arterial fields in the pial circulation of monkeys. The pattern of pressure and flow alteration after the occlusion of the afferent arteries in the neck and of the middle cerebral artery within the skull is described. The initial development of increasing collateral flow appears so rapidly consequent upon the establishment of pressure differentials between adjacent vascular fields as to confirm a casual relationship. The relation of such hemodynamic adjustments to possible later supplementary metabolic factors and the significance of such pressure differentials at the pial surface in regard to the function of the circle of Willis are discussed.