Virtually all organisms manifest circadian (24-hour) rhythms, governed by an ill-defined endogenous pacemaker or clock. Several lines of evidence suggest that the Drosophila melanogaster period gene product PER is a clock component. If PER were central to the time-keeping mechanism, a transient increase in its concentration would cause a stable shift in the phase of the clock. Therefore, transgenic flies bearing a heat-inducible copy of PER were subjected to temperature pulses. This treatment caused long-lasting phase shifts in the locomotor activity circadian rhythm, a result that supports the contention that PER is a bona fide clock component.