The responsiveness of cerebral blood flow to changes in arterial carbon dioxide tension was determined in six monkeys following bilateral superior cervical ganglionectomy. Experiments were conducted 10 to 14 days following the removal of both ganglions using phencyclidine hydrochloride as the anesthetic agent. Following the initial acute experiments, the animals were placed in a sealed environmental chamber for five days at an inspired carbon dioxide level of 6%. The responsiveness to carbon dioxide was repeated following the chronic exposure to carbon dioxide. The animal was killed immediately and the brain removed. The major vessels of the circle of Willis were examined histochemically for the presence of sympathetic nerve fibers. The results of the study demonstrated that: (1) autoregulation was still present, (2) acute exposure to increased levels of carbon dioxide increased flow, and (3) some adaptation of flow occurred following a chronic exposure to 6% carbon dioxide. The carbon dioxide sensitivity of this group of animals was found to be 0.37 cm per second mm Hg-1as compared to a value of 0.94 cm per second mm Hg-1for normal animals. The difference in these two values was significant. It is concluded that the sympathetic nervous system is necessary for the normal responsiveness to changes in arterial carbon dioxide.