Aging alters circadian components such as the free-running period, the day-to-night activity ratio and photic entrainment in behavioral rhythms, and 2-deoxyglucose uptakes and neuronal firing in the suprachiasmatic nucleus (SCN). A core clock mechanism in the mouse SCN appears to involve a transcriptional feedback loop in which Period (Per) and Cryptochrome (Cry) genes play a role in negative feedback. The circadian rhythm systems include photic entrainment, clock oscillation, and outputs of clock information such as melatonin production. In this experiment, we examined clock gene expression to determine whether circadian input, oscillation, and output are disrupted with aging. Circadian expression profiles of rPer1, rPer2, or rCry1 mRNA were very similar in the SCN, the paraventricular nucleus of the hypothalamus (PVN), and the pineal body of young and aged (22–26 months) rats. On the other hand, the photic stimulation-induced rapid expression of Per1 and Per2 in the SCN was reduced with aging. The present results suggest that the molecular mechanism of clock oscillation in the SCN, PVN, and pineal body is preserved against aging, whereas the impairment of Per1 induction in the SCN after light stimulation may result in impaired behavioral photic entrainment in aged rats. J Neurosci. Res. 66:1133–1139, 2001. © 2001 Wiley-Liss, Inc.