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. (C) 2002 Elsevier Science (USA). All rights reserved.