Abstract: Objectives To detect temporal expression profiles of 4 principle clock genes (mBMAL1, mPER1, mCRY1 and mCRY2) in liver, kidney, spleen and testis. Methods Real time PCR assays were employed in this study. Results It indicated that cyclic expressions of clock genes in liver and kidney were both robust and significant (P<0.01, respectively). On the other hand, daily fluctuations of mBMAL1, mPER1, mCRY1 were only mild in spleen (P values equal to 0.008, <0.01, 0.024, respectively), where mCRY2 did not show any significant daily variation at all (P>0.05, respectively). In testis, all of these clock genes did not vary with time, and their abundances were different from other tissues significantly. Conclusions Molecular feedback loops seem to work differently in different tissues, with mPER1 working more efficiently in liver and mCRY1/2 in kidney and spleen, which warrants further studies to elucidate tissue dependant clock machinery.