The brain is supplied by the internal carotid and vertebral arteries that have different regulation with higher CO2 reactivity and larger orthostatic reduction for the internal carotid than the vertebral artery. The different regulation of the two arteries may reflect higher sympathetic innervation of arteries that originate from the internal carotid artery than those of the vertebral artery. The internal carotid artery may contribute to cerebral autoregulation as the vessel dilates during a moderate decrease in MAP while internal carotid artery blood flow is maintained. The arterial CO2 tension (PaCO2) is an important regulator of CBF and also maintenance of central blood volume and cardiac output is important for regulation of CBF. In the present study, internal carotid and vertebral artery blood flow is evaluated using duplex ultrasound in 20 healthy men when MAP is reduced by sodium nitroprusside, a potent short-lasting vasodilator.