Since the introduction of the use of the cranial window by Forbes,1study of the vasomotor reaction of the pial arteries has been productive of a number of important results. Not only the influence of chemical substances on these arteries has been revealed but also the effect of changes in the composition and osmotic concentration of the blood and the effect of the stimulation of various nerves. Simultaneously with the influence which these diverse forces exert on the cerebral arteries, however, there are frequently universal vasomotor reactions and consequent changes in both the arterial and the venous blood pressure; therefore, it is necessary to investigate what effect changes in the endovascular pressure itself have on the pial arteries. Both in this and in several subsequent articles, the question as to the relationship between the arterial blood pressure and the tonus of the pial arteries will be discussed. The reaction