Protease-nexin 1 (PN1), also known as glia-derived nexin, is a protease inhibitor secreted by cultured fibroblasts and glioma cells, with postulated roles in regeneration and the regulation of neurite outgrowth. In this study we have localised the sites of PN1 gene expression in rat brain using in situ hybridisation. As expected, cultured cortical astrocytes contained relatively high levels of PN1 mRNA. However, the mRNA localisation in rat brain suggested that the primary sites of synthesis in the CNS are neuronal. Relatively high levels of PN1 mRNA were found in the olfactory nerve layer of the olfactory bulb, in layer V of the cerebral cortex, in magnocellular neurones of the basal forebrain, and in scattered neurones of the striatum. The results show that PN1 gene expression occurs in discrete populations of neurones in the brain, and suggest that these neurones may therefore play a role in the local regulation of neurite outgrowth.