Glutamate is the primary excitatory transmitter in the hypothalamus. It conveys photic information to the suprachiasmatic nucleus of the hypothalamus, thereby entraining the circadian clock to environmental light cycles. While ionotropic glutamate receptors have been implicated in the transduction of photic information in suprachiasmatic nucleus cells, there is evidence that metabotropic glutamate receptors play a significant modulatory role. We investigated the effects of the metabotropic glutamate agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) on light-evoked phase responses in Syrian hamsters at three phase points circadian time 6, a time when light has no effect on the circadian timing system; circadian time 13.5, when light evokes the maximum phase delay; circadian time 19, the maximum phase advance. We found that ACPD significantly increased the light-evoked phase shift at circadian time 13.5, and had no effect at other phase points tested. These data support a role for metabotropic glutamate receptors in the circadian photic signal transduction system.