Circadian behavioral rhythms in Drosophila depend on the appropriate regulation of at least two genes, period (per) and timeless (tim). Previous studies demonstrated that levels of PER and TIM RNA cycle with the same phase and that the PER and TIM proteins interact directly. Here we show the cyclic expression of TIM protein in adult heads and report that it lags behind peak levels of TIM RNA by several hours. We alsoshow that nuclear expression of TIM depends upon the expression of PER protein. Finally, we report that the expression of TIM, but not PER, is rapidly reduced by light, suggesting that TIM mediates light-induced resetting of the circadian clock. Since both PER and TIM RNA are unaffected by light treatment, the effects of light on TIM appear to be posttranscriptional.