Insulin-like growth factors (IGFs) have numerous actions on neuronal and glial cell function in vitro, although their in vivo roles within the central nervous system (CNS) remain undefined. Levels of IGF-II are high in most rat tissues before the third postnatal week, but rapidly decrease thereafter, except in the brain and spinal cord, where elevated titers are present in the adult. This suggests a function of IGF-II within the CNS. IGF-binding proteins (IGFBPs) modify the type 1 IGF receptor-mediated activity of IGFs, thereby regulating the activities of IGF-II in the CNS. In this study, we use a ribonuclease protection assay, in situ hybridization, and immunohistochemistry to demonstrate that IGF-II and one of the major CNS binding proteins, IGFBP-2, show a striking congruency in their anatomical pattern of expression and localization throughout the adult rat brain. Both proteins are synthesized predominantly in the leptomeninges, choroid plexus, and parenchymal microvasculature, but become localized, remote from the site of synthesis, in the myelin sheaths of individual myelinated axons and in all of the myelinated nerve tracts in the brain, which presumably represents the site of IGF-II bioactivity. The spatial disparity between sites of synthesis and sites of bioactivity suggests a key role for IGFBP-2 in the regulation of IGF-II bioavailability within the brain.