The present study was done to define the morphological characteristics of nenurons in the rat neostriatum. The rats were perfused with 1% paraformaldehyde and 2% glutaraldehyde fixative in phosphate buffer at pH 7.4. Tissues from caudate nucleus were postfixed with 2% osmium tetroxide in phosphate buffer at pH 7.4. and dehydrated in graded ethanol and aceton, and embedded in Epon 812. The polymerization was carried out in a 60℃ oven. The ultrathin sections were contrasted with uranyl acetate and lead citrate, and examined with a Siemens Elmiskop IA. Four morphologically differenttypes of neurons were classified. (1) The large neurons were often elongated or spindle shapes, measured 25~30µm in diameter. They had an indented nucleus and contained large amount of cytoplasm with many stacks of granular endoplasmic reticulum, concentration of ribosomes and rich Golgi cisternae. (2) The medium-sized, polygonal neurons were pyramid or polygonal shapes, measured 14~20µm in diameter. All neurons of this type had a spherical, unindented nucleus and moderate cytoplasm of pale cytoplasm with relatively small amount of granular endoplasmic reticulum and riboromes. (3) The medium sized, ovoid neurons were the most common neostriatal cells. These neurons were ovoid or spherical shapes, measured 12~20µm in diameter. They contained small quantities of Golgi cister in the dark cytoplasm. (4) The small neurons were the least common cells in the striatum. They had a distinctive nucleus, usually with a single indentation and contained little Glogi apparatus and few cisternae of granular endoplasmic reticulum.