The occurrence of retrograde blood flow in the ipsilateral vertebral artery of patients with proximal occlusion of the subclavian or innominate artery has been amply demonstrated by selective angiography. However, since flow artifacts may be induced by the physical or chemical effects of injections of contrast media, it was thought desirable to employ a different method for physiologic demonstration of the reversed flow.In the first case, cardio-green dye was injected into the innominate artery and a dilution curve was recorded from the left brachial artery. The early appearance of the dye (6 seconds) proved that a short circuit existed between the arterial injection and sampling sites, and confirmed the authenticity of the reversed vertebral flow shown in previous angiograms.In the second patient, who had symptoms of cerebral ischemia during exercise of the left (affected) arm, dye injected into the left carotid artery did not appear early in the brachial artery of the resting left arm. When it was injected during a period of intense hyperemia of the left arm, a fraction of the dye did appear early. This confirmed that, when the demand of the arm increased, blood was shunted to it not only from the vertebral-basilar system but also from the carotid system.The importance of anatomic factors was evident in another case in which absence of the anticipated retrograde flow was explained by the fact that the vertebral artery arose directly rectly from the aorta and did not communicate with the affected subclavian artery.These observations illustrate how radiographic and hemodynamic technics may usefully be combined to assess the effects of acquired diseases of the major blood vessels.