Fibroblast growth factors (FGFs) are potent growth factors with roles in the brain ranging from development to adult plasticity. FGF-5 is a newly described member of the fibroblast growth factor family. In order to evaluate a possible role of FGF-5, we have examined the locus of synthesis of FGF-5 in the rat brain. We have used in situ hybridization of 35S-labelled RNA probe complementary to FGF-5 mRNA. FGF-5 mRNA was present in neurons in select regions of the rat brain. FGF-5 mRNA expression was particularly intense in the olfactory bulb within periglomerular elements and the mitral cell layer. The primary olfactory cortex also showed a robust expression of FGF-5 mRNA mostly within layer II. In the hippocampal formation, the greatest labelling of FGF-5 mRNA occurred in hippocampal pyramidal cells within subfields CA3 and secondarily within subfields CA1, CA2 and CA4. The dentate gyrus granule cells displayed a modest hybridization signal. The cerebral cortex (neocortex) showed a light labelling throughout its rostro-caudal extent mostly within external layers. The entorhinal cortex showed a higher FGF-5 mRNA expression as compared to the neocortex and signal appeared more intense in layer II. In general, FGF-5 mRNA was shown to be localized mostly in limbic structures, suggesting that FGF-5 may play a role in limbic system function or dysfunction.