Administration of the dopamine (DA) antagonist haloperidol leads to the development of behavioral hypersensitivity as well as enhanced neuronal growth when striatal extracts from these animals are incubated with mesencephalic cultures. For determining if alterations in neuronal growth also occur in vivo, the ultrastructure of the neuropil in the dorsolateral quadrant of the striatum from rats treated (24 days) with haloperidol (1.25 mg/kg) was examined by electron microscopy. Haloperidol-treated rats developed statistically significant behavioral hypersensitivity relative to vehicle-treated controls (p < 0.01). Evaluation of the neuropil revealed that haloperidol treatment enhanced, relative to vehicle-treated controls, the overall number of synaptic boutons by 9% (p < 0.01). The number of perforated synaptic profiles as well as the number of double synapses was increased by 20 and 50%, respectively, although this increase was not statistically significant. The number of myelinated axons remained unchanged, while the number of dendritic spines was increased by 21% (p < 0.05). These data suggest that chronic haloperidol treatment enhanced the growth and possible sprouting of presynaptic neurons and also induced postsynaptic plastic changes. These ultrastructural changes may contribute in part to hypersensitivity behaviors.