Although transcriptional regulation is a major force in generating circadian oscillations of clock molecules, posttranscriptional mechanisms also contribute to molecular rhythms. Applying novel transgenic period-luciferase constructs in transgenic Drosophila, the authors show that sequences within per's 5'-untranslated region mediate posttranscriptional regulation at the RNA level. Further mapping suggests that the relevant sequences for the correct phasing of period mRNA expression are located within the first intron. The results are consistent with a clock-regulated temporal stabilization of period mRNA during its daily upswing in the morning. This process is inferred to depend on a function of the PERIOD and TIMELESS proteins, and could further contribute to the observed delay between RNA and protein accumulation. Similarly, applying timeless-luciferase constructs led to the demonstration that regulatory elements for proper temporal timeless expression are present in a 4 kb promoter fragment and in sequences within the first intron. The results establish that, for normal rhythmicity, expression of clock genes requires regulation at the transcriptional, posttranscriptional, and posttranslational levels.