The circadian clock is an endogenous mechanism that generates rhythms with an approximately 24-h period and enables plants to predict and adapt to daily and seasonal changes in their environment. These rhythms are generated by molecular oscillators that in Arabidopsis (Arabidopsis thaliana) have been shown to consist of interlocking feedback loops involving a number of elements. An important characteristic of circadian oscillators is that they can be entrained by daily environmental changes in light and temperature. Previous work has shown that one possible entrainment point for the Arabidopsis oscillator is the light-mediated regulation of expression of one of the oscillator genes, CIRCADIAN CLOCK ASSOCIATED1 (CCA1). In this article, we have used transgenic plants with constitutive CCA1 expression to show that light also regulates CCA1 transcript stability. Our experiments show that CCA1 messenger RNA is relatively stable in the dark and in far-red light but has a short half-life in red and blue light. Furthermore, using transgenic plants expressing chimeric CCA1 constructs, we demonstrate that the instability determinants in CCA1 transcripts are probably located in the coding region. We suggest that the combination of light regulation of CCA1 transcription and CCA1 messenger RNA degradation is important for ensuring that the Arabidopsis circadian oscillator is accurately entrained by environmental changes.