Drosophila melanogaster bearing mutations in the DCO gene, which encodes the major catalytic subunit of cAMP-dependent protein kinase (PKA), displays arrhythmic locomotor activity strongly suggesting a role for PKA in the circadian timing system. This arrhythmicity might result from a requirement for PKA activity in photic resetting pathways, the timekeeping mechanism itself, or downstream effector pathways controlling overt behavioral rhythms. To address these possibilities, we examined the protein and mRNA products from the clock gene period (per) in PKA-deficient flies. The per protein (PER) and mRNA products undergo daily cycles in the heads and bodies of DCO mutants that are indistinguishable from those observed in control wild-type flies. These results indicate that PKA deficiencies affect the proper functioning of elements downstream of the Drosophila timekeeping mechanism. The requirement for PKA in the manifestation of rhythmic activity was preferentially greater in the absence of environmental cycles. However, PKA does not appear to play a universal role in output functions because the clock-controlled eclosion rhythm is normal in DCO mutants. Our results suggest that PKA plays a critical role in the flow of temporal information from circadian pacemaker cells to selective behaviors.