The effect on the cerebral blood flow (CBF) of stimulation of the cervical sympathetic nerve trunk and of intracarotid infusion of levarterenol was explored in anesthetized baboons at varying levels of arterial carbon dioxide pressure. Sympathetic stimulation reduced CBF, and this reduction was more pronounced during hypercapnia; levarterenol reduced CBF by 17% during hypercapnia. On morphological grounds and from physiological data it is argued that there is a dual control of the cerebral circulation—the extraparenchymal vessels being influenced by the sympathetic nervous system, while the intraparenchymal vessels are under local intrinsic metabolic regulation. The pial vessels are possibly influenced by both systems.