In order to examine the possible trophic, nontransmitter role of monoaminergic fibers in the adult CNS, synaptic structures were examined in different laminae of the somatosensory cortex of the rat following a p-chlorophenylalanine (PCPA)-induced decrease of monoamine. Synaptic densities were reduced in a dose-dependent fashion by 30-50% in the target area of monoamine fibers following four injections of PCPA made over a 1-week period. Although serotonin- and tyrosine hydroxylase-immunopositive profiles were frequently observed in all laminae of the cerebral cortex, only a few such profiles had the morphology of synapses. Therefore, virtually all of the reduction in synaptic structures following PCPA treatment involved nonmonoaminergic fibers.