In a supine position the main drainage from the brain is through the internal jugular vein (IJV) but the vertebral veins (VV) become important during orthostatic stress when the IJV is “collapsed”. To indicate whether this shift in venous drainage from the brain during orthostatic stress reflects that flow from the part of the brain served by the internal carotid artery (ICA) becomes drained by VV, a correlation analysis between ICA and VV blood flow was performed during supine rest and when 9 subjects (5 men) were seated. To appreciate inter-individual variation, determination of vascular flow was carried out with hypocapnia via hyperventilation and inhalation of 6 % carbon dioxide (CO2). During supine rest change in ICA blood flow in response to manipulation of CO2 was reflected in IJV blood flow (P<0.001), but not in VV blood flow (P=0.121). When seated, both ICA and IJV blood flow decreased (P=0.004 and P=0.002), while vertebral artery (VA) blood flow did not change (P=0.784). In addition, changes in VV blood flow in response to manipulation of CO2 became associated with that in ICA blood flow (P<0.001) to an extent that the correlation tended to be higher (r=0.828) than between IJV and ICA blood flow (r=0.790). These results support that venous drainages from the anterior part of the brain is shifted to the posterior veins when IJV collapses and raises the question whether a VV siphon supports cerebral blood flow during orthostatic stress.