Man would have syncope while standing upright if it were not for certain adaptive mechanisms which tend to maintain a constant cerebral blood flow. In the standing position the amount of blood present in the upper part of the body is decreased, first, because the vascular bed in the portion of the body below the heart is dilated by a high hydrostatic pressure1and, second, because the high capillary pressure causes an increased filtration of fluid from the blood stream and a decrease in plasma volume.2This decrease in blood volume in the upper part of the body is compensated for by vasoconstriction and by increase in heart rate. In a group of subjects first described by Bradbury and Eggleston3in a discussion of postural hypotension, these reactions failed to compensate for the pooling of blood and there was a striking fall in arterial pressure when the