Dihydroergotamine (DHE) is the treatment of choice in aborting the acute attack of migraine. Although its efficacy has been known for 40 years, its mechanism of action is still disputed. Data regarding the site of action of dihydroergotamine may provide an insight into its mechanism of action and thus identify a locus of potentially abnormal pathophysiology in migraine. By using in vitro and ex vivo autoradiographic techniques, the localization of specific binding sites for 3H-dihydroergotamine in the cat brain has been examined. Binding was seen in the dorsal horn of the cervical spinal cord, in the medulla, associated with the nucleus of the tractus solitarius, area postrema, and descending spinal trigeminal nucleus, and in the mesencephalon and the cerebral cortex. The highest density of binding sites was found in the dorsal and medial raphe nuclei of the midbrain. Furthermore, these same brain regions were also labeled after intravenous administration of 3H-dihydroergotamine. It is important that the brain areas specifically labeled are key nuclei involved in cranial pain transmission, suggesting that dihydroergotamine may act at these central sites in migraine.