The neostriatum of 20 adult humans was examined in Golgi-Kopsch and rapid Golgi preparations. At least five types of neurons and four types of afferent fibers are described. Neurons of medium size with spine-rich dendrites (spiny type I) are the most frequent type. These cells exhibit a greater morphological diversity than those previously studied in the monkey. Also, quantitative data show that, compared to the monkey, spiny type I neurons in man have a greater somal size and dendritic field radius. Although the types of spines are similar, the mean density and radial distribution of spines along dendrites differ in the two species. Morphologic features of the axon, which is usually long with extensive collateral branches, suggest that more than one process from the axon may be efferent. Medium to large neurons with sparsely spined dendrites (spiny type II) differ from type I neurons in having a poorer branching and greater radial spread of their dendrites and a lower density of spines. They also differ from spiny type I neurons in their distribution and relative proportion of various spine types. The axon of the spiny type II neuron is long and has collaterals which are poorly arborized in comparison to those of spiny type I cells. Aspiny neurons are of medium (aspiny type I) and large (aspiny type II) size. They have varicose, curved dendrites and a short axon which arborizes mostly within the dendritic field. A group of smaller neurons with more variable dendritic morphology is also seen. Observations suggest that in the human brain the proportion of medium-sized aspiny neurons and small neurons may be greater than in other species.