The effect of postural blood pressure changes on the blood flow velocity in the middle cerebral artery (MCA) was studied with transcranial Doppler sonography in 45 patients with intact circulatory reflexes and in 30 cases with postural hypotension. In an upright position on the tilt-table the patients with intact cardiovascular reflexes showed a small decrease of the MCA flow velocity of 3% (1.5 cm/sec), whereas there was found a marked reduction in the cases with postural hypotension. In the group with an asympathicotonic response (n = 14) there was an average decrease of 24% (13 cm/sec) and a 28% drop of mean arterial blood pressure. The lower percentage of reduction of the cerebral blood flow velocity can be explained as an effect of cerebral autoregulation. In the group with a sympathicotonic response (n = 16) a surplus reduction of the flow velocity of 17% (7 cm/sec) was seen, compared to a 7% decrease in blood pressure. This corresponds with the results of experimental animal studies which showed an impaired cerebral autoregulation during increased sympathico-adrenal discharge.