Circadian transcriptional rhythms mediated by BMAL1/CLOCK play essential roles in behavioral and physiological circadian rhythms. Abundant studies have shown that BMAL1/CLOCK-mediated transcription rhythms are generated through positive and negative feedback loops. However, roles and functions of intracellular signal transduction pathways for regulation of BMAL1/CLOCK-mediated circadian transcription rhythms are still unclear. Therefore, we have tried to identify intracellular signal transduction pathways that play regulatory roles in BMAL1/CLOCK-mediated transcription. To do this, we examined the effects of various selective inhibitors of intracellular signal transduction pathways on BMAL1/CLOCK-mediated circadian transcription rhythms in NIH3T3 cells. Cells were treated with 50% horse serum for 2 hrs (serum shock) to induce circadian transcription rhythms, and then examined expression rhythms of BMAL1/CLOCK-target genes. We found that pharmacological inhibition of phosphoinositide 3-kinase (PI3K) significantly modulated circadian oscillation of mRNA levels for several BMAL1/CLOCK-targeted clock genes including dbp. For example, the inhibition of PI3K strongly decreased the expression level of Dbp mRNA. Consistently, luciferase reporter assay revealed that the inhibition of PI3K also decreased the activity of Dbp promoter. Thus we showed that PI3K plays modulatory roles in BMAL1/CLOCK-mediated transcription rhythms. We are now examining effects of PI3K knockdown (p110α and p110β; major isoforms of PI3K) on BMAL1/CLOCK-mediated gene expression.