To explore the role of serotonergic system in seasonal affective disorder (SAD), we compared growth hormone (GH) responses to a challenge with a novel 5-HT1D receptor agonist sumatriptan between 11 patients with SAD and nine healthy controls. Of the 11 patients with SAD, nine had repeat sumatriptan challenge following treatment with light therapy. The results showed that GH responses were significantly blunted during winter depression in patients with SAD compared to healthy controls. The GH responses normalized following treatment with light therapy to similar levels in controls. The results of this study provide a support for the role of serotonergic system in pathophysiology of SAD and in the mechanism of action of light therapy.