IGF-I and IGF-II are ubiquitously expressed growth factors that have profoun d effects on the growth and differentiation of many cell types and tissues, incl uding cells of the CNS. In biologic fluids, most IGFs are bound to one of six IG F binding proteins (IGFBPs 1-6). Increasing evidence strongly supports a role f or IGF-I in CNS development, as it promotes neuronal proliferation and survival . However, little is known about IGF-I and its homolog IGF-II and their carrie r proteins, IGFBPs, during the neonatal period in which brain size increases dra matically, myelination takes place, and neurons show limited capacity to prolife rate. Herein, we have determined the concentrations of IGF-I, IGF-II, IGFBP-I , and IGFBP-3 in cerebral spinal fluid (CSF) samples that were collected from children who were 1 wk to 18 y of age. The concentrations of IGF-I, IGFBP-I, and IGFBP-3 in CS F from children < 6mo of age were significantly higher than in older children, w hereas IGF-II was higher in the older group. This is in contrast to what is obs erved in the peripheral circulation, where IGF-I and IGFBP-3 are low at birth and rise rapidly during the first year, reaching peak levels during puberty. Hig her concentrations of IGF-I, IGFBP-I, and IGFBP-3 in the CSF of very young ch ildren suggest that these proteins might participate in the active processes of myelination and synapse formation in the developing nervous system. We propose t hat IGF-I and certain IGFBPs are likely necessary for normal CNS development du ring critical stages of neonatal brain growth and development. IGF-I and IGF-II are ubiquitously expressed growth factors that have profoun d effects on the growth and differentiation of many cell types and tissues, incl uding cells of the CNS. In biologic fluids, most IGFs are bound to one of six IG F Increasing evidence strongly supports a role f or IGF-I in CNS development, as it promotes neuronal proliferation and survival. However, little is known about IGF-I and its homolog IGF-II and their carrie rinsin, we have determined the concentrations of IGF-I, IGF-II, IGFBP-I, The concentrations of IGF-I, IGFBP-I, and IGFBP-3 in CS F from children <6 mo of and IGFBP-3 in cerebral spinal fluid (CSF) samples that were collected from children who were 1 wk to 18 y of age. age were significantly higher than in older children, w hereas IGF-II was higher in the older group. This is in contrast to what is obs erved in the peripheral circulation, where IGF-I and IGFBP-3 are low at birth and rise rapidly during the first year, reaching peak levels during puberty. Hig her concentrations of IGF- I, IGFBP-I, and IGFBP-3 in the CSF of very young ch ildren suggest that these proteins participate participate in the active processes of myelination and synapse formation in the developing nervous system. We propose t hat IGF-I and certain IGFBPs are likely necessary for normal CNS development du ring critical stages of neonatal brain growth and development.