In a man with orthostatic and effort syncopes due to primary dysautonomia, we measured cerebral blood flow (CBF)--by the 133-Xenon inhalation method--in supine and in sitting positions, and after the i.v. administration of Acetazolamide, a potent cereal vasodilator. Heart rate (HR) and blood pressure (BP) were monitored over the 24 hours by a non-invasive device. The CBF was normal in supine position and significantly reduced when the patient was sitting. Despite che sympathetic denervation, good response to acetazolamide infusion was seen. BP changed with the position of the subject according to gravity, and HR was unresponsive to orthostatic and effort stimuli.