Interlocking transcriptional loops and regulated protein degradation are the principal mechanisms involved in the generation of self-sustaining circadian rhythms in many organisms. In Arabidopsis the first proposed regulatory transcriptional loop involved the transcription factors CIRCADIAN CLOCK ASSOCIATED (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) and the pseudo-response regulator TIMING OF CHLOROPHYLL A/B BINDING PROTEIN (TOC1/PRR1). Recent findings indicate that the TOC1 homologues PRR7 and PRR9 might also be involved in transcriptional regulatory loops with CCA1 and LHY. In this study we show that the overexpression of PRR7 in Arabidopsis leads to severely compromised circadian rhythms. These transgenic lines display significantly reduced levels of CCA1 and LHY RNA, providing further evidence for a transcriptional feedback loop between PRR7 and these transcription factors. In addition, we show that the PRR7 protein is phosphorylated in a circadian regulated manner and that its levels are post-translationally regulated by both diurnal and circadian mechanisms. The Arabidopsis circadian oscillator is therefore likely to be entrained to light/dark cycles both through transcriptional and post-transcriptional mechanisms.