We examined the ultrastructural development of the lateral geniculate nucleus (LGN) in postnatal tree shrews to distinguish which features, if any, show a correlation with the development of cell layers. Our data indicate that synaptogenesis has begun at birth (PO) which is prior to the development of cell layers. At postnatal day 8 (P8), when laminar borders can be distinguished, the majority of synaptic profiles are still immature. Although some mature synaptic profiles can be identified at P8 and especially at P15, complex synaptic arrangements characteristic of adults are absent at that time. Growth cones are present at all 3 ages but are less prevalent at P8 and P15 than at birth and appear to be present in slightly higher concentrations in the interlaminar spaces. These results suggest that LGN cell layer formation does not correlate with the beginning of synaptogenesis but that it may correlate with an increase in growth cones in the interlaminar spaces.