In order to investigate the post-transcriptional regulation of Period1 (Per1), the 3(')-untranslated region (3(')UTR) of mouse Per1 (mPer1) mRNA was characterized. In addition to high similarity between human and mouse Per1 3(')UTRs, AU-rich element and differentiation control element were found in both species. Transient transfection assays using LUC::mPer1 3(')UTR fusion genes revealed that the mPer1 3(')UTR repressed its own expression in a post-transcriptional manner. The region critical for this translational down-regulation was confined to nucleotide positions 322-517. These results suggest that the mPer1 3(')UTR could be involved in the generation of time lag between the transcriptional and translational products of mPer1 in the suprachiasmatic nucleus.