Casein kinase Iϵ (CKIϵ) and casein kinase Iδ (CKIδ) phosphorylate clock oscillating mPER proteins, and play a key role in the transcription (post)translation feedback loop that generates circadian rhythm. In the present study, the expression profiles of CKIϵ and CKIδ mRNAs were examined in the mice clock center, suprachiasmatic nucleus (SCN). Moderate levels of CKIϵ and CKIδ mRNAs were constantly expressed in the SCN in both light:dark and constant dark conditions. This finding supports the hypothesis that CKI may form a constant threshold to the nuclear entry of mPER proteins as in the Drosophila homologue, double-time. Further, we demonstrated that the light exposure at subjective night induced a delayed increase in CKIϵ and CKIδ mRNAs in the SCN. CKIϵ and CKIδ proteins may play a role on light-induced phase-shift. J. Neurosci. Res. 64:612–616, 2001. © 2001 Wiley-Liss, Inc.