Cerebral blood flow was measured in four subjects with chronic idiopathic autonomic insufficiency using85Kr inhalation, cerebral A-V samples for scintillation counting, and O2content determination. Blood pressure was varied by up or down tilting and L-norepinephrine infusion. In three patients with typical Shy-Drager syndrome and preganglionic denervation, CBF regulation to pressure and PaCO2change was intact. In a fourth patient with postganglionic denervation, CBF autoregulation was absent to changes in blood pressure but was preserved normally to increases or decreases in PaCO2CBF and CMRO2, at rest were normal (52.6 cc and 3.1 cc). Hyperventilation to PaCO234.3 mm failed to restore autoregulation to increased blood pressure. The results imply a functional role for postganglionic autonomic fibers in CBF autoregulation.