The protein product (PER) of the Drosophila clock gene, period (per), is involved in a molecular feedback loop in which PER inhibits the transcription of its own mRNA. This feedback causes the PER protein to cycle in a circadian manner, and this cycling in specific regions of the brain (the presumed location of the central pacemaker) is responsible for the rhythmicity of locomotor activity and possibly eclosion. PER has also been detected in several nonneural tissues in the abdomen, but whether PER exhibits free-running and light-sensitive cycles in any of these tissues is not known. In this study, the authors assayed the spatial and temporal distribution of a PER-reporter expressed in transgenic flies carry ing a per-lacZ construct, which was shown to cycle in per-expressing brain cells. The authors demonstrate that this PER-reporter fusion protein cycles in the Malpighian tubules, showing first cytoplasmic accumulation, which is then followed by translocation of the signal into the nucleus. To test whether this rhythm was controlled by the brain, flies were decapitated and assayed for 3 days after decapitation. Expression patterns of PER-reporter in decapitated flies were nearly identical to those in intact flies reared in normal light-dark cycles, reversed light-dark cycles (phase shifted), and constant darkness. These results suggest that the Malpighian tubules contain a circadian pacemaker that functions inde pendently of the brain.