Insulin-like growth factor-1 (IGF-1) stimulates the proliferation and maturation of neuroglia in vitro. To further investigate its role in gliogenesis, in situ hybridization was utilized to determine whether IGF-1 mRNA was expressed in the subventricular zone (SVZ) of the postnatal mouse forebrain. The SVZ is a transient germinal zone and in the neonate is the principle source of oligodendroglia for myelinating fiber tracts of the forebrain. Strong hybridization signal was detected over cells in the SVZ at postnatal day (PND) 4, the earliest time point examined. Positive signal persisted in the SVZ at PND 8, however, the number of IGF-1-labeled cells declined rapidly during the second postnatal week. IGF-1 mRNA was not uniformly distributed throughout the SVZ and the majority of labeled cells were located within its so-called 'border' region. In contrast to the SVZ, IGF-1 mRNA-expressing cells were only rarely found in forebrain fiber tracts. IGF-1 transcripts were not detected in ependymal lining or choroid plexus of the lateral ventricle. In light of its known gliotrophic activity, the localization of IGF-1 mRNA in the SVZ suggests that locally produced IGF-1 may act as a mitogen or differentiation-inducing agent during gliogenesis.