Syrian hamsters were fitted with guide cannulas stereotaxically aimed at the suprachiasmatic nuclei (SCN) and housed in constant darkness in cages equipped with running wheels. Animals received 300-nl injections of either vehicle, CNQX, or MK-801 into the region of the SCN 5 min before a brief (10 min at 20 lx) light exposure. Local administration of either 1 mM CNQX or 1 mM MK-801 at circadian time (CT) 18 significantly inhibited light-induced phase advances (vehicle = 52 +/- 9 min; CNQX = 12 +/- 7 min; MK-801 = 12 +/- 5 min; P < 0.05 relative to vehicle+light group). The effects of both drugs were reversible and dose related. Injection of 1 mM MK-801 at CT13.5 resulted in a 71% inhibition of light-induced phase delays (vehicle = -51 +/- 6 min; MK-801 = -15 +/- 5 min; P < 0.05), while CNQX failed to significantly inhibit light-induced phase delays (-39 +/- 10 min). Local administration of either 1 mM CNQX or MK-801 into the SCN region reduced the number of Fos-immunoreactive cells relative to vehicle-injected controls by approximately 32 and 44%, respectively (vehicle = 951 +/- 79; CNQX = 643 +/- 135, P < 0.05; MK-801 = 533 +/- 143, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)