In Golgi preparations of the adult monkey (Macaca mulatta) local interneurons and two varieties of principal neurons, radiating and elongated fusiform, are found in the subthalamic nucleus. The cell bodies of the radiating neurons have a few delicate, somatic spines some of which are occasionally bilobed and trilobed. Five to eight dendritic trunks give rise to branching, tapering dendrites, which may extend for over 400 microns. These dendrites are much thinner than the dendrites in the globus pallidus and the substantia nigra. Some neurons have many and some neurons have few dendritic spines. When numerous the dendritic spines are concentrated on the dendritic trunks and proximal dendrites. The relatively few elongated fusiform neurons are found not only in the capsule but also in the center of the nucleus. Most dendrites emerge from the opposite poles of their smooth surfaced cell bodies. They have a few dendritic spines. Some of these dendrites extend for more than 750 microsn. In 1-micron thick plastic sections lipofuscin granules are present in some but not all principal neuron cell bodies of the monkey (Macaca mulatta); but these granules are present in all principal neuron cell bodies of the pig-tail monkey (Macaca nemestrina) and of the squirrel monkey (Saimiri sciureus). The local interneurons have small cell bodies and a few relatively long undulating dendrites. The dendrites have bulbous dendritic appendages of varying complexity and beaded axon-like processes. The dendritic appendages and axon-like processes are more numerous distally and on the distal ends of the dendrites they form complex entanglements. Axons coming from the cell body have not been observed. The cell bodies of the local interneurons are identified in cresyl violet stained sections of the monkey (Macaca mulatta), in 1-micron thick plastic sections and electron micrographs of the squirrel monkey (Saimiri sciureus). They have relatively large nuclei surrounded by a thin rim of cytoplasm rich in polyribosomes.