The intracellular distribution of measles virus inclusion bodies in persistently infected human cells (AV3A1/MV) changed markedly following continuous exposure to 3', 5' cyclic adenosine monophosphate (cAMP). When assayed by immunofluorescence, the number of cells with intranuclear virus inclusions increased from 5 to 10% to 80 to 90% after exposure to 1 mM-cAMP for 4 days. Exposure of cells to cAMP also resulted in a twofold increase in the average number of inclusions in invaded nuclei. Similar but less pronounced changes occurred in cells treated with inducers of adenylate cyclase and an inhibitor of phosphodiesterase. Examination of cAMP-treated cells by electron microscopy indicated that viral inclusion bodies consisted of typical helical nucleocapsids. No evidence of nucleocapsids crossing the nuclear membrane (through nuclear pores) was found.