We investigated photic response of the concentration of vasoactive intestinal polypeptide (VIP) and neuropeptide Y in the rat suprachiasmatic nucleus (SCN) by enzyme immunoassay (EIA). The content of VIP in the SCN did not show circadian rhythms in constant darkness (DD). Under light-dark (LD) condition, VIP contents decreased over the course of the light period and then recovered during the dark period. When the light was continuously delivered to rats, VIP levels were monotonically decreased and did not return to the basal level. Accordingly, VIP in the SCN may code photic information on duration. On the other hand, there is a daily bimodal pattern in NPY content in the SCN under light-dark conditions. When rats were exposed to continuous light, the NPY level steadily increased and reached a peak in 2 h before returning to a basal level. The amount of increase did not depend on duration of light exposure. Thus, NPY in the SCN may code visual information on transitions, which is different from that conveyed by VIP. These results indicate that the two peptides act on a different stage of photic processing and may mediate distinct photic information to the circadian pacemaker.