Paired specimens of the neostriatum were taken from monkeys at zero (newborn), one, two, four, eight, and 16 weeks of age, and prepared for Golgi impregnations and electron microscopy. Light microscopy shows that in the first postnatal week, the structure contains the five neuronal types and four categories of afferent axons described in the adult, as well as some cells too undifferentiated to classify. Most neurons exhibit immature dendritic features, including local enlargements, terminal growth cones with filopodia, and filiform processes. In spiny type I cells, various levels of maturity may coexist in regions of a single dendrite, in different dendrites of the same neuron, and among individual cells. Spine density increases progressively with age, but the relative distribution of spine types remains about the same. Spiny type II neurons show some decline in spine density, and generally mature sooner than spiny type I cells. The long axons of spiny neurons have varicosities which disappear at about eight weeks. In younger animals (newborn and one week), the dendrites of aspiny neurons (types I, II, and III) may have a "spiny" appearance, exhibiting many spine-like and filiform processes. Concurrently, the short axons vary in degree of arborization from very immature to well developed. Electron microscopy corroborates the developmental features recognized in the Golgi material: dendritic and axonal growth cones, filopodia and varicosities, as well as various stages of maturation in somata and dendrites. Degenerating elements, mostly of an axonal nature, are seen up to eight weeks. The synapses which reach maturity at birth are of the asymmetric axospinous type, in which the axonal profile contains small round vesicles, and of the symmetric axodendritic class, with the presynaptic elements having pleomorphic vesicles. Some synapses are slower to mature and appear at one to eight postnatal weeks. These include those made by profiles with pleomorphic vesicles, forming either symmetric contacts with somata and axon initial segments, or asymmetric contacts with spines. The same applies to the asymmetric axodendritic synapses made by elements containing small round vesicles. Finally, profiles containing large round or flat vesicles are the latest to participate in mature synapses formation. Findings indicate that a considerable degree of qualitative and quantitative change takes place in the monkey neostriatal neuropil during early postnatal development, especially in the first eight-week period.