Internal and external carotid blood flow was simultaneously measured in monkeys. The experimental technique, which was validated by observations on response of the intracranial and extracranial vasculature to CO2 inhalation, hypertension, sympathetic stimulation, and the cerebral vasodilator, papaverine, appears to be an improved approach to the study of migraine mechanisms in the animal model. lntracarotid serotonin was shown by this method to constrict both the internal and external carotid arteries. This effect was prevented by intracarotid methysergide, which itself produced only a transient constriction of the same vessels.