The brain of cocaine administrated rats was histopathologically investigated by light and electron microscopy. The examined regions were the nucleus accumbens core and shell, and the striatum pars dorsolateral. Morphometric analyses concerning the density of dendritic spines and the number of large aspiny neurons were also performed. A total of 15 male rats, 8-week-old, were used as experimental animals. Cocaine HCL was intraperitoneally injected into the rat at a dose of 15 mg/kg/day for 2 weeks. These rats were divided into three groups based on examination purposes such as staining techniques for histology and electron microscopy. As a control group, a total of 15 rats were injected with 0.9% saline solution. In the cocaine administrated rats, the density of dendritic spines on medium spiny neurons in the above three regions were increased compared with that of the control group. At the ultrastructure level, the dendritic spines were frequently observed in the experimental group. The large aspiny neurons of the striatum pars dorsolateral in the experimental group showed a tendency to increase in number. These findings were statistically supported by morphometric analyses. From these obtained results, it is suggested that the increase of dendritic spines and of large aspiny neurons might be induced by cocaine administration, and these facts might cause the alteration of synaptic connectivity pattern in mesocorticolimbic dopmine circuitry.