The suprachiasmatic nucleus-pineal system acts as a neuroendocrine transducer of seasonal changes in the photoperiod by regulating melatonin formation. In the present study, we have investigated the extent to which the photoperiod entrains the nonself-cycling oscillator in the Sprague-Dawley rat pineal. For this purpose, the 24-h expression of nine clock genes (bmal1, clock, per1, per2, per3, cry1, cry2, dec1 and dec2) and the aa-nat gene was monitored under light-dark 8 : 16 and light-dark 16 : 8 in the rat pineal by using real-time RT-PCR. The 24-h pattern of the expression of only per1, dec2 and aa-nat genes was affected by photoperiod. In comparison with the short photoperiod, the duration of elevated expression under the long photoperiod was elongated for per1 and shortened for dec2 and aa-nat. For each of the genes, photoperiod-dependent variations partly persisted under constant darkness. Therefore, the pineal clockwork appears to memorize the photoperiod of prior entrained cycles. The findings of the present study indicate that the nonself-cycling oscillator of the rat pineal is entrained by photoperiodic information and therefore that it participates in seasonal timekeeping.