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In mammals, the thalamic intergeniculate leaflet (IGL) conveys behavioral (non-photic) phase-resetting information to the circadian clock of the suprachiasmatic nucleus. Here we report a 24 h fluctuation in in vivoserotonin release in the hamster IGL region, peaking at night. Novel wheel exposure at midday, a stimulus that can reset circadian phase, activates the release of serotonin in the IGL region. In addition, electrical stimulation of the dorsal raphe nucleus acutely increases serotonin release the IGL region, confirming a functional serotonergic projection from this nucleus to the IGL. Collectively, these findings suggest that behavioral andor circadian changes in dorsal raphe nuclear activity could modulate serotonin-mediated activities of the IGL.