The suprachiasmatic nuclei (SCN) have been identified as a pacemaker for many circadian rhythms in mammals. Photic entrainment of this pacemaker can be accomplished via the direct retino-hypothalamic tract (RHT). Glutamate is a putative transmitter of the RHT. In the present study it is demonstrated that glutamate injections in the SCN cause phase shifts of the circadian activity rhythm of the hamster. In contrast, glutamate injections outside the SCN or vehicle injections inside the SCN did not affect the circadian phase. These data suggest that glutamate could be involved in photic entrainment of the circadian pacemaker.