Cyclic adenosine monophosphate (cAMP)-dependent protein kinase, labeled with fluorescein and rhodamine on the catalytic and regulatory subunits, respectively, was injected into Aplysia sensory neurons either in culture or in intact cell clusters. Energy transfer between the subunits, a measure of cytosolic cAMP concentration ([cAMP]), and compartmentation of the dissociated subunits were monitored by confocal fluorescence microscopy. Bath application of serotonin produced a much greater elevation of [cAMP] in the processes than in the central bodies of the neurons. The resulting gradients must drive a sizable centripetal flux of cAMP because direct microinjection of cAMP showed that it diffused readily. Perinuclear increases in [cAMP] slowly caused the translocation of the freed catalytic subunit into the nucleus to an extent proportional to the percentage of its dissociation from the regulatory subunit.