Synapse formation and maturation were examined in the rat dorsal lateral geniculate nucleus (dLGN) from birth to adulthood. Examination of animals, whose ages were closely spaced in time, showed that the maturation of the synaptic organization of the nucleus takes place chiefly during the first 3 weeks of postnatal life. This period of maturation may be divided into 3 broad stages. During the first stage, which spans the first 4 days of life, there are only a few immature synapses scattered throughout the nucleus; occasionally aggregates of 3 or 4 synapses are encountered. Dendrodendritic synapses first appear at the end of this stage. The second stage, which lasts from the end of the first stage through day 8, is characterized by intensive synaptogenesis as well as extensive growth and degeneration. For the first time, large boutons resembling retinal terminals form multiple synaptic contacts with dendrites and dendritic protrusions; these synaptic arrangements are partially covered by glial processes. A feature characteristic of the developing dLGN during the first 2 postnatal weeks, and particularly during the second stage, is the presence of membrane specializations that resemble vacant postsynaptic densities. These specializations, which may be unapposed or opposite another neuronal process, decrease in frequency as the number of synapses increases. It is not known whether these densities are converted to synapses or whether they result from loss of presynaptic elements. The third stage in the process of synaptogenesis, which spans a period between days 10 and 20, is characterized by myelination and by the diminution of growth cones, degenerating profiles and vacant postsynaptic densities. There is also a very significant increase in the number and maturation of synapses including synaptic glomeruli. However, it is not until the end of this stage that synapses appear qualitatively indistinguishable from synaptic arrangements identified in adult animals.