In both Drosophila melanogaster and the honeybee Apis mellifera, cyclic adenosine monophosphate (cAMP)-dependent processes have been implicated in mechanisms of learning. This study characterizes the type II cAMP-dependent protein kinase (PKAII), the major target of cAMP in adult animals. In both species, PKAII is restricted to neuronal tissue, in which it accounts for more than 90% of total PKA activity. Although the intensity of PKAII immunoreactivity differs between distinct brain regions, labeling is detectable in all neuropiles and most somata. While the visual neuropiles, the antennal lobes, and structures of the central brain exhibit intermediate immunostaining, the mushroom bodies show high labeling and contain a three- to fourfold higher PKA activity compared to other neuropiles. Since the mushroom bodies are central sites of olfactory learning mediated via cAMP-dependent signaling, the modulatory functions of transmitters on PKA activity in Kenyon cells from the honeybee were tested. Agents which elevate cytoplasmic Ca2+ levels have no effects on PKA activity in cultured Kenyon cells. Dopamine, serotonin, and octopamine, however, cause an increase in PKA activity in Kenyon cells. The modulation of PKA activity by octopamine, the putative transmitter of the unconditioned stimulus in associative olfactory learning in the honeybee, together with the findings on the central role of the cAMP cascade in Drosophila mushroom bodies, suggests a major implication of PKAII-mediated phosphorylation in learning and memory in both Drosophila and Apis.