The distribution and density of asymmetric synapses including biocytin-labelled corticostriatal synapses of the rat neostriatum were examined at postnatal day 10 (P10), P15, P21 and in adults. The density of asymmetric synapses in the adult neostriatum (28.0 synapses/100 μm²) was significantly greater than that in neonates at P15 (14.4 synapses/100 gm²) and P10 (11.5 synapses/100 μm²), but not at P21 (24.2 ± 1.5 synapses/ 100 μm²). The increased density of asymmetric synapses in the adult neostriatum was due primarily to an increase in the number of axospinous synapses. The density of axospinous synapses was greatest in adults (22.3 synapses/100 μm²) and significantly less at P21 (15.3 synapses/100 μm²), P15 (5.9 synapses/100 μm²), and P10 (2.0 synapses/100 gmm²). The density of axodendritic synapses, however, remained similar at all ages (adult, 3.9 ± 1.1 synapses/100 μm²; P21, 6.0±1.2 synapses/ 100 μm²; P15, 5.7 ± 0.8 synapses/100 μm² or P10, 7.2± 1.3 synapses/100 μm²). Iontophoretic injection of biocytin into the lateral frontal agranular cortex produced labelling of corticostriatal afferents which formed asymmetric synapses in the neostriatum. The distribution of termination sites of biocytin-labelled corticostriatal boutons showed a pattern of development similar to the unlabelled asymmetric synapses. The present study shows that the increase in the overall number of asymmetric synapses over the first three postnatal weeks can be attributed to an increase in the density of asymmetric axospinous synapses. During the same period little change is noted in the number or density of asymmetric axodendritic synapses. These changes in excitatory synaptic input to medium spiny neurons may explain some of the previously described electrophysiological differences noted between the neonatal and adult neostriatum.