In order to assess the hypothesis that excitatory amino acids (EAA) are involved in the transmission of light information from retina to suprachiasmatic nucleus (SCN) and pineal via the retinohypothalamic tract (RHT), we have determined whether injections of EAA agonist into SCN could mimic the suppressive effects of light pulse on pineal melatonin production, and whether pretreatment with antagonists could block effects of light pulse in the intact rat. Injection of the EAA agonist N-methyl-D-aspartate (NMDA: 1.0 mM; 0.5 microliter) into the SCN suppressed plasma melatonin level and pineal N-acetyltransferase activity. The pretreatment with D-aminophosphonovalerate (D-APV: 2.5 or 10 mM; 2.0 microliters) or N-[1-(2-thienyl)-cyclohexyl]-piperidine (10 mM; 2.0 microliters) which are NMDA type receptor antagonists blocked the suppressive effect of the light pulse (3.0 Ix for 2 min), while the pretreatment with neither vehicle nor L-APV (optic isomer APV: 10 mM; 2.0 microliters) could block the effect of light. Alpha-D-glutamyl-amino-methylsulfonate (10 mM; 2.0 microliters or 25 mM; 2.0 microliters), which is a relative antagonist for non-NMDA type receptor, had no effect, either. These results suggest that EAA is involved in the transmission of light information through RHT and that in rat SCN EAA operates at the NMDA type receptor on the SCN.