Whole brains dissected from third-instar larvae ofDrosophila melanogasterincrease their cAMP content when incubated with octopamine, dopamine and serotonin, especially in the presence of theophylline, a phosphodiesterase inhibitor. The most pronounced rise in cAMP was produced by forskolin. When brains prelabelled with [32P]orthophosphate were subjected to these treatments, the increased labelling of 27.5-kDa protein(s) of pI 6.3-6.6 (substrate A) was observed in all cases, as revealed by two-dimensional gel electrophoresis and autoradiography. Labelling of substrate A was enhanced without any drug addition in brains of thedunceM11mutant strain, which has abnormally high basal cAMP levels. The labelling of substrate A was also promoted by the potassium channel blocker 4-aminopyridine. The data suggest that inDrosophilalarval brain the 27.5-kDa protein is a target of cAMP-triggered phosphorylation and may be related to potassium channel(s).