We used transmission electron microscopy to study ultrastructural changes accompanying synaptogenesis in the fetal and postnatal mouse visual cortex (primary visual cortex). Immunostaining and DiI diolistic assay were also employed in order to evaluate synaptophysin expression and dendritic spine development. Nascent synapses were seen as early as E15, although these were immature and were composed of a presumed presynaptic terminal with pleiomorphic vesicles in the vicinity of a partner cell body or projection. The postsynaptic plasmalemma remained unspecialized and the gap between pre- and post-synaptic plasmalemmas was only 5-10nm, significantly narrower than the mature synaptic cleft. With increasing age there was gradual thickening of both the pre- and post-synaptic membranes, with widening of the synaptic cleft to 15-20 nm. Ultrastructurally mature synapses were not seen until P7; at this time both Gray's type I and II could be observed. Synaptogenesis correlated with the development of synaptic spines and synaptophysin expression. Because synapse maturation was synchronous with dendritic spine differentiation, synaptic specialization may be dependent on dendritic spine maturation and the expression of presynaptic vesicle components. In the meantime, the study also indicated that the synaptogenesis was connected with the development and maturation of neocortex.