The mammalian circadian clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus and in most peripheral tissues. Clock genes drive the biological clock. However, circadian expression variations of the human clock genes are still unclear. In this study, we analyzed the daily variations of mPer2 and mClock mRNA expression in both the mouse SCN and liver to evaluate the central and peripheral alterations in the rodent clock genes. We also examined whether there are the daily variations of the clock genes hPer2 and hClock in human peripheral blood mononuclear cells (PBMCs). The daily variation of mClock and mPer2 mRNA expression in mouse SCN and liver were determined at ZT2, ZT6, ZT10, ZT14, ZT18 or ZT22. We isolated PBMCs from 9 healthy volunteers at 9:00 and 21:00 and examined the expression of hPer2 and hClock mRNA by RT-PCR analysis. The animals exhibited a robust daily rhythm in the RNA levels of mPer2 in the SCN and liver (P<0.01, respectively). In humans, hPer2 mRNA expression also had daily variation, and the hPer2 mRNA levels at 9:00 were significantly larger than those at 21:00 (P<0.01). While, the Clock mRNA in both mice and humans exhibited no daily variation. These findings suggest that the variation in hPer2 mRNA expression may be useful for assessing human peripheral circadian systems.