We have previously isolated insulin-like growth factor binding protein-4 (IGFBP-4) from media conditioned by a neuronal cell line and have detected IGFBP-4 mRNA in selected regions of the developing and adult rat brain by Northern blot analysis. In this study, the ontogeny and regional distribution of IGFBP-4 expression were determined by in situ hybridization histochemistry. While IGFBP-4 mRNA expression at embryonic day 15 was restricted to choroid plexus primordium and meninges, by embryonic day 20 IGFBP-4 mRNA was also localized in the basal ganglia. In the postnatal rat, at days 1 and 5, IGFBP-4 was also present in the meningeal cell layer surrounding the developing cerebellum and in the hippocampal formation. The distribution of IGFBP-4 mRNA in the adult brain was considerably more widespread. The principal areas where IGFBP-4 mRNA was detected were the cerebral cortex (layers II and IV), olfactory peduncle (anterior olfactory nuclei), limbic system (hippocampus and amygdala), thalamus and basal ganglia, as well as choroid plexus and meninges. The widespread and persistent expression of IGFBP-4 is in marked contrast with IGFBP-2, the other IGFBP in the brain, whose localization by in situ hybridization is reported to be restricted to choroid plexus and meninges. The spatial pattern of IGFBP-4 expression in areas known to either overlap, be adjacent to, or project to regions that express the IGFs or their receptors may reflect a role for IGFBP-4 as a modulator of IGF action in the brain. © 1993 Wiley-Liss, Inc.