Circadian clocks consist of three elements: entrainment pathways (inputs), the mechanism generating the rhthmicity (oscillator), and the output pathways that control the circadian rhythms. It is difficult to assign molecular clock components to any one of these elements. Experiments show that inputs can be circadianly regulated(1-3) and outputs can feed back on the oscillator(4,5), Mathematical simulations indicate that under- or overexpression of a gene product can result in arrhythmicity, whether the protein is part of the oscillator or substantially Dart of a rhythmically expressed input pathway(6). To distinguish between these two possibilities, we used traditional circadian entrainment protocols(7,8) on a genetic model system, Neurospora crassa.