The importance of the circadian clock for the control of behavior and physiology is well established but how and when it develops is not fully understood. Here the initial expression pattern of the key clock gene period was recorded in Drosophila from embryos in vivo, using transgenic luciferase reporters. PERIOD expression in the presumptive central-clock dorsal neurons started to oscillate in the embryo in constant darkness. In behavioral experiments, a single 12-h light pulse given during the embryonic stage synchronized adult activity rhythms, implying the early development of entrainment mechanisms. These findings suggest that the central clock is functional already during embryogenesis. In contrast to central brain expression, PERIOD in the peripheral cells or their precursors increased during the embryonic stage and peaked during the pupal stage without showing circadian oscillations. Its rhythmic expression only initiated in the adult. We conclude that cyclic expression of PERIOD in the central-clock neurons starts in the embryo, presumably in the dorsal neurons or their precursors. It is not until shortly after eclosion when cyclic and synchronized expression of PERIOD in peripheral tissues commences throughout the animal.