Investigations by Fog1and Forbes, Nason and Wortman2have demonstrated that the pial arteries react to a fall in blood pressure by dilatation. This reaction was shown to be independent of the way in which the fall in pressure was produced. It was noted also by these authors that the same arteries react to a rise in blood pressure by constriction, but no detailed description of the latter reaction was given. During recent experiments with epinephrine,3I have made observations which confirmed these earlier findings, but in view of the various conclusions reached by other investigators, it seemed desirable to carry out further experiments, using the method of direct observation. Heretofore, conclusions as to cerebral vascular reactions have been drawn largely from data obtained by indirect methods. By all such methods the changes in caliber of the vessels are inferred, not observed. Therefore, even if data were