A conserved transcription-translation negative feedback loop forms the molecular basis of the circadian oscillator in animals. Molecular interactions within this loop have been relatively well characterized in vitro and in cell culture; however, in vivo approaches are required to assess the functional significance of these interactions. Here, regulation of circadian gene expression was studied in vivo by using transgenic reporter mouse lines in which 6.75 kb of the mouse Period1 (mPer1) promoter drives luciferase (luc) expression. Six mPer1-luc transgenic lines were created, and all lines express a daily rhythm of luc mRNA in the suprachiasmatic nuclei (SCN). Each mPer1-luc line also sustains a long-term circadian rhythm of luminescence in SCN slice culture. A 6-h light pulse administered during the early subjective night rapidly induces luc mRNA expression in the SCN; however, high luc mRNA levels are sustained, whereas endogenous mPer1 mRNA levels return to baseline, suggesting that posttranscriptional events mediate the down-regulation of mPer1 after exposure to light. This approach demonstrates that the 6.75-kb mPer1 promoter fragment is sufficient to confer both circadian and photic regulation in vivo and reveals a potential posttranscriptional regulatory mechanism within the mammalian circadian oscillator. N early all organisms express circadian (Ϸ24-h) rhythms in behavior, physiology, and cellular activity. In mice, Drosophila, Neurospora, and cyanobacteria, extensive studies indicate that the basic molecular circadian mechanism consists of a transcription-translation feedback loop (1, 2). Recent reviews describe the mammalian model in detail (3, 4); briefly, the transcription factors CLOCK and BMAL1 (also known as MOP3) activate transcription of the mouse Period (mPer1 and mPer2) and Cryptochrome (mCry1 and mCry2) genes. The PER and CRY proteins accumulate and translocate into the nucleus where they inhibit the activity of CLOCK and BMAL1. The turnover of the inhibitory PER and CRY proteins then leads to a new cycle of activation by CLOCK and BMAL1.Several genes in this transcriptional pathway exhibit circadian rhythms of expression, but they differ in characteristics of rhythmic expression such as circadian phase and response to light (3). For example, peak mRNA expression of mPer1, mPer2, and mCry1 occurs at different times in the suprachiasmatic nucleus (SCN), the site of the circadian pacemaker in mammals (5-7). However, the protein products of these three genes accumulate in SCN neurons around the same phase (8-10). In addition, a light pulse administered during the early subjective night leads to the rapid induction of mPer1, slower induction of mPer2, and no induction of mCry1 (11)(12)(13)(14). Clearly, regulated circadian gene expression remains an important component of the circadian mechanism.Transcriptional regulation of circadian promoter activity has been addressed initially in cell culture. In cell transfection͞ luciferase reporter assays, the CLOCK and BMAL1 proteins dimerize and bind thre...