A morphometric study of the synapses on dendritic shafts and spines was performed in the rat caudate nucleus and the CAI area of the hippocampus under chronic haloperidol treatment. In the nucleus caudatus, the synaptic density on dendritic shafts increased by 83% and those on spines by 53%. Most of the parameters measured in axospinous synapses were significantly increased: the area of presynaptic axon terminals (20%), the number of mitochondria per axon terminal (51%), the length of active zone (11%), the area of postsynaptic density (23%), and the perimeter of postsynaptic density (12.5%). The area of postsynaptic spines showed no changes. In the synapses on dendritic shafts, the area of presynaptic terminals decreased (31%), the area of mitochondria per terminal decreased (40%), the length of active zone increased (14%), and other parameters were unchanged. There were no significant differences in the same parameters measured in the hippocampus. The data are discussed as morphological correlates of behavioral supersensitivity and dopamine D2 receptor up-regulation.