The subthalamic nuclei of 9 rhesus monkeys, ranging in age from newborn to 17 weeks, were examined at the light and/or electron microscopic levels, using computer assisted quantitative methods. The volume of the structure does not change significantly over the period of study. The mean cross-sectional area of neuronal somata, however, decreases by 33%, and most markedly during the first month. This is paralleled by a similar change in the mean area of cell nuclei but the perikaryon/nucleus ratio increases steadily after the first week. There is an overall decline in total cell numbers from newborn to 17 weeks. Ultrastructural features include dendritic growth cones in the neonatal monkey, and signs of axonal degeneration during the entire period. In addition to conventional axosomatic and axodendritic synapses, there are also synaptic junctions between vesicle-containing profiles which are seen only after the first month. The distributions of plaque diameters were reconstructed by the Coupland stereologic method from linear measurements of synaptic profiles and used to calculate synaptic densities and estimate the total number of synapses. This number is stable during the first postnatal month, declines markedly in the second month, and to a lesser degree thereafter, reaching a value of 55% of that at birth by 16 weeks. Findings indicate the occurrence of substantial changes within the subthalamic nucleus during the first 4 postnatal months, the most prominent of which is a marked synapse elimination.