Examination of the nestriatum of monkeys prepared by the Golgi-Kopsch perfusion method revealed the presence of at least 6 neuronal types. The spiny type I is medium size with a high density of dendritic spines. The axon extends well beyond the dendritic field and gives off many collaterals. The spiny type II is either medium or large size, has long thick dendrites with a relatively low density of spines, and an axon similar to that of the previous type but with fever collaterals. The aspiny type I is medium size with varicose dendrites and a thin axon arborizing in the immediate vicinity of the soma. The aspiny type II is large, with many thick and thin varicose dendrites. The aspiny type III is medium size with smooth dendrites and an axon ramifying profusely within the dendritic field. The neurogliform cell is small with many branching processes. Findings indicate that the neostriatum has 2 distinct types of spiny neurons with long axons (spiny I and II), some of which may contribute to the efferent system. There are also 2 (aspiny I and III) or perhaps as many as 4 categories (aspiny I, II, III and neurogliform) of typical Golgi type II cells. Large neurons belong to 2 separate populations, one with dendritic spines and a long axon (large version of spiny II), and one with varicosities and presumably a short axon (aspiny II). A realistic interpretation of neurophysiologic data on the neostriatum must take into account all cell types instead of the current view of considering it as a pool of interneurons with few output cells.