The fibroblast growth factor family of polypeptides currently consists of nine structurally-related members. Cloning of the mouse homologue of the latest reported member of the family, FGF-9, is described in this study. Mouse Fgf9 exhibits a high level of sequence conservation with the human, rat and Xenopus counterparts. Of note is the lack of a hydrophobic signal peptide at the N-terminus of the coding sequence. The protein, however, appeared to be secreted by producer cells since a significant quantity of the protein could be purified from the culture supernatant of transfected cells. Members of the FGF family are known to bind to cell surface tyrosine kinase receptors (FGFRs) to elicit a variety of physiological responses. These receptors themselves form a family of four structurally-related tyrosine kinases and each FGF member commonly has the ability to bind several members of the FGFR family. By using in vitro plate binding assays, FGF-9 is shown in this study to bind specifically to two FGFR members: FGFR2b and FGFR3c. To further study the potential functional role of FGF-9, its expression pattern in the mouse embryo was examined by both RNase protection and RNA in situ hybridization analyses. The transcript was detected in a variety of embryonic tissues: the germinal epithelium of the central nervous system, the mesonephric cords, the somites, the gut primordia and the developing eye and ear, suggesting that the gene may have multiple roles during development. In addition, the potential involvement of FGF-9 in mediating adult brain functions was examined by double RNA in situ hybridization analysis of the distribution of both Fgf9 and Fgfr3 transcripts in the adult mouse brain. Most apparent areas of co-localization are the olfactory bulb and cerebral cortex. The two transcripts are also shown to have distinct distribution patterns in the cerebellum.