The effect of chronic administration of chlorpromazine (CPZ) on the striatal synaptic vesicles was quantitatively investigated with an electron microscope. Six rats, three controls and three experimentals, which received 20 mg/kg b.wt. of CPZ daily for 15 months, were sacrificed and a total of 300 axo-dendritic spine synapses (50 synapses per animal) were randomly taken and divided into three zones with a width of 160-467 nm on electron micrographs (final magnification 60,000). The first zone was adjacent to the presynaptic membrane in the region of the synaptic cleft. The second and third were farther away from the first zone. In these zones we measured the vesicle density, vesicle size and vesicle elongation ratio (the shortest diameter/the longest diameter) with the following results: In both the CPZ-administered rats and the controls the vesicle number per unit area of axoplasm (vesicle density) was smaller in Zone III than in Zones I and II, while those in the latter two zones did not differ from each other. However, there was no significant difference in the vesicle number in any zone between the control and experimental animals. The vesicles in every zone were significantly larger in the CPZ-administered animals than in the controls. There were no conclusive findings on the vesicle elongation ratio (vesicle shape).