THE ROLE of vasomotor nerve fibers in the regulation of the caliber of intracranial arteries remains an important clinical and experimental problem. Numerous clinical reports have attested to the benefits of cervical sympathectomy by surgical or chemical means in the treatment of demonstrable or presumed cerebral circulatory disorders. 1 Yet quantitative methods have frequently, but not invariably, failed to demonstrate an alteration of the total cerebral blood flow following stimulation of the cervical sympathetic trunk or block of the stellate ganglion. 2 It is generally recognized, however, that such results do not invalidate anatomic and physiologic evidence 3 of a vasomotor innervation of certain pial and intracerebral arteries. Reapportionment of cerebral blood flow might conceivably occur as the result of stimulation of the cervical sympathetic trunk or stellate-ganglion block without a concomitant, detectable alteration in the total blood flow. This investigation was designed to study the hypothesis by means of