Period1 (Per1) is one of several clock genes driving the oscillatory mechanisms that mediate circadian rhythmicity. Per1 mRNA and protein are highly expressed in the suprachiasmatic nuclei, which contain oscillator cells that drive circadian rhythmicity in physiological and behavioral responses. We examined a transgenic mouse in which degradable green fluorescent protein (GFP) is driven by the mPer1 gene promoter. This mouse expresses precise free-running rhythms and characteristic light induced phase shifts. GFP protein (reporting Per1 mRNA) is expressed rhythmically as measured by either fluorescence or immunocytochemistry. In addition the animals show predicted rhythms of Per1 mRNA, PER1 and PER2 proteins. The localization of GFP overlaps with that of Per1 mRNA, PER1 and PER2 proteins. Together, these results suggest that GFP reports rhythmic Per1 expression. A surprising finding is that, at their peak expression time GFP, Per1 mRNA, PER1 and PER2 proteins are absent or not detectable in a subpopulation of SCN cells located in the core region of the nucleus.