Dysfunction of serotonergic system has long been implicated in the pathophysiology of migraine. Numerous studies have shown a reduction in platelet and urinary serotonin with elevation of its metabolites during the phases of a migraine attack. Moreover, intravenous administration of sumatriptan, a kind of selective 5-HT 1B/1D receptor agonist, aborts migraine attacks. Since serotonin and sumatriptan, hardly pass through the blood brain barrier, it has been thought that the vascular or peripheral serotonergic system might contribute to migraine pathophysiology. On the other hand, several clinical and experimental reports have suggested that a low brain 5-HT disposition may facilitate migraine. Together with the fact that serotonin mobilizers, such as fenfluramine and reserpine, have been shown to be effective in migraine prophylaxis when used for an extended period, these observations indicate that low brain serotonin disposition may also be involved in migraine pathophysiology, especially in the chronic phase of migraine. However, involvement of the serotonergic nervous system of the brain parenchyma in the pathophysiology remains unclear. To address that, we prepared an experimental model of migraine by generation of cortical spreading depression, characterized by spreading of neuronal/glial membrane depolarization accompanied by temporal elevation of the cerebral blood flow (CBF) throughout the cerebral cortical hemisphere both in rats which underwent pharmacological treatment for degeneration of serotonergic neurons in the dorsal raphe nucleus and in untreated rats. We show here that 1) the significant degeneration of serotonergic neurons in the dorsal raphe nucleus and serotonergic fibers in the cerebral cortex was observed in treated rats; 2) the spreading velocity of the CBF changes was significantly increased in these rats; 3) the calculated width of the depolarization wave was significantly extended in these rats. These results indicate that the dorsal raphe serotonergic neurons modulate cortical spreading depression and might be involved in migraine pathogenesis via a similar mechanism.