This aspect of the regulation of the cerebral circulation has been under intensive investigation for the past several years. Many uncertainties have been clarified, but a satisfying answer to the important question of what role neurogenic influences play in the physiological regulation of cerebral blood flow (CBF) remains elusive. Since this subject has been reviewed repeatedly (7, 18, 46, 53), the present paper concentrates on the most recent evidence directly pertinent to the participation of vasomotor nerves in physiological adjustments of the cerebral circulation. Adrenergic fibers originating in the ipsilateral superior cervical ganglion innervate both pial and intracerebral vessels (42). The effects of cerebral adrenergic nerves are substantially less pronounced than those seen in other vascular beds. In the cat stimulation of the cervical sympathetic nerves produced pial arteriolar vasoconstriction. The average response was mod­ est, ranging from 7-12% (29, 69). Larger pial arteries responded more vigorously than smaller ones. In two studies (51, 69) from the same labora­ tory no significant response was· detectable in the smaller « 100 JLm in diameter) arterioles. The responses of CBP and cerebral vascular resistance (CVR) to electrical stimulation of the cerebral sympathetic nerves depend on the species studied. The most pronounced and consistent responses were obtained in monkeys (20) and rabbits (55); in the latter, the response was