Rhythmic oscillations of the PER protein, the product of theDrosophila period (per) gene, in brain neurons of the adult fly are strongly involved in the control of circadian rhythms. We analyzed temporal and spatial expression patterns of three per–reporter fusion genes, which share the same 4 kb regulatory upstream region but contain increasing amounts ofper’s coding region fused in frame to the bacteriallacZ gene. The fusion proteins contained either the N-terminal half (SG), the N-terminal two-thirds (BG), or nearly all (XLG) of the PER protein. All constructs led to reporter signals only in the known per-expressing cell types within the anterior CNS and PNS. Whereas the staining intensity of SG flies was constantly high at different Zeitgeber times, the in situ signals in BG and XLG flies cycled with ∼24 hr periodicity in the PER-expressing brain cells in wild-type andper 01 loss of function flies. Despite the rhythmic fusion-gene expression within the relevant neurons ofper 01 BG flies, their locomotor activity in light/dark cycling conditions and in constant darkness was identical to that of per 01 controls, uncoupling protein cycling from rhythmic behavior. The XLG construct restored weak behavioral rhythmicity to (otherwise)per 01 flies, indicating that the C-terminal third of PER (missing in BG) is necessary to fulfill the biological function of this clock protein.