In adult animals corticothalamic (CT) axons pass through the thalamic reticular nucleus (Rt) where they give off collateral branches innervating the Rt neurons. The postnatal development of CT projections from the somatosensory cortex, with particular reference to the branching pattern within Rt, ventrobasal (VB) and posterior (PO) nuclei, was investigated in the rat with anterograde tracing. Biotinylated dextran-amine (BDA) was iontophoretically injected into the somatosensory cortex of rats ranging from postnatal day (P) 0 to P30. At P1 most of the cortical axons traversed unbranched Rt and terminates in VB and PO, whereas at P3 they formed rudimentary branches in these nuclei. From P6 to P9 a progressive increase in the amount of dense clusters of terminal arborizations was evident in Rt, and by the second postnatal week more complex arborizations with a clear topographic arrangement were observed in Rt, VB and PO. Our findings indicate that CT fibers show a quantitative increase both in R1 and in somatosensory thalamic nuclei during the first postnatal week, although their terminal arborizations are however still incomplete. The pattern of collateralization of CT projections achieves an adult configuration at the end of the second postnatal week.