RearingDrosophila melanogasterin constant darkness (DD) for multiple generations disrupts the circadian activity rhythm of adults. In order to determine under what conditions normal rhythms can be initiated, DD-rearedDrosophila(either the wild type or theperiodshort[perS] mutant) were exposed to light either as embryos, thirdinstar larvae, or adults. Exposing DD-reared flies to light as embryos or larvae had no effect, while exposing them as adults fully restored normal rhythms inperSand partially restored normal rhythms in the wild type. The percentage of adults with normal rhythms was not significantly different between animals given a 1-h pulse of light as adults and animals given two LD cycles as adults. LD-reared and DD-reared animals given 2 LD cycles were synchronous. In the latter, offset of activity followed the LD transition (CT 12) by 2–6 subjective hours inperSand 2–3 subjective hours inper+. Circadian rhythms did not exhibit phase coherence in the other treatments.