The mammalian hypothalamic suprachiasmatic nucleus (SCN) harbors an intrinsic circadian oscillator driving a variety of endogenous rhythms. SCN activity is entrained to environmental lighting conditions by photic information from the retina. Light-induced phase shifts involve cAMP and Ca2+ as second messengers and are linked to transcriptional and translational processes. Using in situ hybridization with a cDNA fragment of the cAMP responsive element modulator (CREM) isoform inducible cAMP early repressor (ICER) we demonstrate a light-induced upregulation of CREM mRNA in rat SCN during the second half of the night. Notably, ICER is the only member of the CREM family which is transcriptionally inducible. We suggest that a stimulus-induced upregulation of ICER expression can inhibit cAMP-inducible genes in rat SCN via its trans-repressing potency.