Because the projections into the robust nucleus of the archistriatum (RA) are thought to play important roles in song learning and sexual differentiation of the zebra finch (Poephila guttata), quantitative electron microscopic techniques were used to measure the development of synaptic input to the neuropil of RA in this species. Two nuclei [hyperstriatum ventrale pars caudalis (HVc) and lateral magnocellular nucleus of the anterior neostriatum (IMAN)] that send projections to RA were lesioned at each of three different ages: 25 d after hatching, 53 d, and adulthood. In tissue from RA processed for conventional electron microscopy, lesion-induced degeneration was used to identify synapses from either HVc or IMAN. Axosomatic synapses were excluded from analysis. In control (unlesioned) animals, the density of synapses in neuropil increased slightly between days 28 and 56 and remained constant thereafter. Because of a large increase in the volume of RA, the total number of synapses in neuropil of RA tripled between days 28 and 56 and decreased significantly between day 56 and adulthood. The density and total number of synapses in neuropil originating from HVc increased significantly between days 25 and 53, but did not change significantly thereafter. In contrast, the density and total number of synapses from IMAN decreased significantly between days 25 and 53 and did not change thereafter. Presynaptic terminals from IMAN were larger than those from HVc. These data demonstrate that the most rapid phase of song learning is accompanied by a major rearrangement of synaptic contacts into RA that stem from HVc and IMAN.