A variety of observations have suggested that the dopamine and serotonin systems may play a role in the pathophysiology and treatment of major mental disorders of childhood, adolescence and early adulthood. A recent triple immunofluorescence study has demonstrated a convergence of serotonin and dopamine fibers onto both pyramidal cells and GABAergic interneurons in the rat medial prefrontal cortex (mPFCx). These findings are consistent with the results of an electrophysiological study conducted in another laboratory that suggested such a relationship exists in the pyriform cortex of the rodent brain. During postnatal development, the dopamine system shows a progressive ingrowth of fibers into this region that continues until the early adult period. In contrast, GABAergic neurons appear to complete their postnatal maturation by the fourth postnatal week (the early post-weanling period). As dopamine fibers infiltrate the rat mPFCx, they progressively increase their interaction with neural elements within the neuropil and with the cell bodies of both pyramidal cells and GABAergic interneurons. This process appears to be influenced by the serotonin system, since lesioning of the nucleus raphe dorsalis during the neonatal period results in a significant increase of dopamine fibers. This finding suggests that lesions of the serotonin system induce plasticity of the cortical dopamine system; however, it is not known whether this inferred suppressive effect of serotonin fibers occurs at brainstem levels or within the mPFCx itself. Taken together, these various studies suggest that the convergence of dopamine and serotonin fiber systems on intrinsic cortical neurons shows considerable plasticity during postnatal life that could theoretically contribute to the development of 'miswired' circuits in individuals with neuropsychiatric disorders.