The globus pallidus of 20 monkeys ranging in age from newborn to 4 months was examined in Golgi-impregnated material and ultrastructurally. There was no discernible difference between the lateral and medial segments of the structure. At the light microscope level, all neuronal types described in the adult are found at birth. The most common, the large fusifom cell, shows initial signs of immaturity such as blunt protrusions and dendritic dilations at bifurcation points, as well as growth cones, filopodia, and filiform processes. These features become more rare with age, and by 4 months, the neurons appear fully mature save for the terminal dendritic arborizations which are still underdeveloped. From the earliest ages examined, the large globular cells and the interneurons are more mature than the previous type. The afferent radial fibers of striatal origin are observed from birth, but they are grouped in bundles only after 8 weeks. The density of their climbing branches increases over time, reaching a mature appearance by 16 weeks. Afferents entering from the ventral surface do not yet show clusters of varicosities at 2 weeks. At the latter age, plexuses of fine beaded fibers are already seen covering large extensions of the nucleus. The fine structure correlates well with the Golgi material. The basic features of the neuropil are present at birth, albeit with immature characteristics such as the incomplete covering of the dendrites with axonal boutons and the low level of myelination of the radial fibers. Growth cones and profiles with signs of degeneration are observed during the first month. In the early ages examined, most dendrites show large varicosities and protrusions, some of which are spinelike and can be postsynaptic to multiple terminals. The other dendritic type, with only an occasional axodendritic synapse, is also seen from birth and increases in size as a function of time. The type I axonal boutons, of probable striatal origin, are quite immature at birth, and their characteristic interdigitations are seen only after the first week. The type II, III, IV, and V boutons appear mature at all ages examined but crest synapses formed by the type III terminals are observed in the later stages of the study. Finally, postsynaptic vesicle-containing profiles are present at 4 weeks, but triadic synaptic arrangements are formed only by 16 weeks.(ABSTRACT TRUNCATED AT 400 WORDS)