The aim of this study was to test the role of glutamatergic neurotransmission in photic entrainment of the circadian oscillator of the suprachiasmatic nuclei (SCN) in the Syrian hamster. The response of the oscillator to a brief pulse of light was assessed using two independent indices, the phase shift of the free-running activity rhythm, and the photically induced expression of the immediate-early gene c-fos within neurons of the SCN. The behavioural and the cellular responses to light were compared in animals which received intra-cerebroventricular (icv) infusions into the region of the SCN of either a vehicle solution or a solution of γd-glutamyl-glycine (DGG), a competitive antagonist at both N-methyl-d-aspartate (NMDA) and non-NMDA types of glutamate receptor. Infusions of vehicle or DGG (200 nmol) were given 10 min before presentation of a 15-min light pulse at either circadian time (CT) 14 or CT20 (onset of activity defined as CT12). As anticipated, animals treated with vehicle and light at CT14 exhibited phase delays in the activity rhythm, whereas animals treated at CT20 exhibited phase advances. Central infusion of DGG prior to a light pulse at CT14 blocked the phase-delaying effect of light, and DGG delivered before a light pulse at CT20 markedly attenuated the phase-advancing effect of light. In a separate group of animals, the expression of the immediate-early gene c-fos was assessed by immunocytochemical staining for its protein product Fos. Exposure of vehicle-infused animals to light at CT14 caused extensive expression of c-fos throughout the retinorecipient region of the SCN. However, when the light pulse was preceded by icv fusion of DGG at a dose which would block the phase-shifting response to light, the total number of neurons immunopositive for Fos was significantly reduced (∼50%) and the expression was confined to a restricted area of the dorsolateral SCN. The precise correlation between the effects of glutamatergic blockade upon both the behavioural and the cellular responses of the circadian system to light demonstrates that effective glutamatergic neurotransmission within or adjacent to the SCN is a necessary component of the mechanism which mediates photic entrainment of the circadian clock. The results further demonstrate a pharmacological and anatomical compartmentalization of the retinorecipient zone of the SCN, consistent with the view that retinal afferents to the ventral region employ glutamate as a transmitter, whereas more dorsal input may be dependent upon non-glutamatergic (DGG-insensitive) pathways.