Individual neostriatal-matrix spiny neurons were stained intracellularly with biocytin after intracellular recording in vivo, and their axons were traced into the globus pallidus (GP), entopeduncular nucleus (EP), and/or substantia nigra (SN). The locations of the neurons within the matrix compartment of the neostriatum (NS) were established by immunocytochemical counterstaining of sections containing the cell bodies using antibodies for calbindin- D28K. This allowed nearly complete visualization of the axonal projections of single NS neurons. On the basis of their intrastriatal axonal arborizations, matrix spiny neurons could be divided into 2 types. One type, which was the more common, had local axonal arborizations restricted to the region of the dendritic field, often with axon collaterals arborizing within the dendritic field of the cells of origin. A second, less common, cell type in the matrix had local axon collaterals distributed widely in the NS. Among matrix neurons with restricted local collateral fields, 3 subtypes could be distinguished on the basis of their efferent axonal projections. Type I cells projected only to the GP. Type IIa cells projected to the GP, EP, and SN pars reticulata. Type IIb cells projected to the GP and SN but not to the EP. The shapes and densities of the GP arborizations varied in the 3 cell types, with the cells projecting only to the GP (type I) projecting more heavily and filling a larger volume there than type II cells. The dendrites and intrastriatal axon collaterals of 3 subtypes were similar in morphology. The class of matrix spiny neurons with intrastriatal axon collaterals distributed widely in the NS were observed to project to the GP. Projections beyond the GP were not identified for this cell type, but could not be ruled out. Somatodendritic morphologies of neurons did not differ according to the projection site. These results demonstrate that NS matrix spiny cells are more heterogeneous in their efferent projection patterns than previously suspected on the basis of retrograde axonal tracing and immunocytochemical studies. As predicted by those previous studies, there is a class of matrix neurons that projects only to the GP. Presumably, these cells contain enkephalin. Cells projecting to the SN and EP, and so presumably containing substance P, give off a small projection to the GP, as well, and differ in their collateralization patterns within the 3 major target nuclei.