The aim of the present study is to examine whether serotonergic terminals of the rat brain are induced to contain dopamine by L-DOPA administration. In rats that received intraperitoneally L-DOPA plus a peripheral decarboxylase inhibitor, we showed by use of a double-labeling immunofluorescence method that dopamine was localized in serotonergic fibers of the striatum and cerebral cortex as well as in serotonergic cell bodies of the midbrain raphe nuclei. In normal rats, no dopamine was detected in the serotonergic fibers or cell bodies. The finding suggests that a part of the administered L-DOPA may enter the serotonergic terminals and be converted to dopamine in the rat brain.