The effects of daily administration of haloperidol for 16 weeks on the structure of layer VI in medial prefrontal cortex of rat was performed at the light and electron microscopic levels. At the light microscopic level, no difference in either the size or the density of neurons was observed. At the electron microscopic level, the mean dendritic calibre of haloperidol-treated rats was twice that observed in control animals, but this was due to a selective loss of small-calibre dendritic profiles. Rats treated with neuroleptic also showed a reduction in axon terminals with asymmetric postsynaptic membrane specializations, which, in control animals, were preferentially associated with small-calibre dendritic profiles. These small-calibre dendritic profiles were found to be spines rather than small terminal dendritic shafts. An increase in axon terminals showing no membrane specialization on larger dendritic profiles also occurred in rats treated daily with the neuroleptic. The data suggest the possibility that haloperidol may have induced a relocation of asymmetric terminals from resorbed spinous processes to larger dendritic branches with the concomitant loss of their postsynaptic membrane specialization.