Basic fibroblast growth factor (FGF), its mRNA and the mRNA that encodes for its receptor have all been localized in the rat subfornical organ (SFO). Basic FGF is widely distributed throughout the SFO; it is present in neurons, in the vascular basement membrane of lateral blood vessels (but not those within the SFO) and in ependymal cells surrounding the SFO. Results of in situ hybridization show that the expression of basic FGF mRNA is detected throughout the organ. Similarly, the expression of flg, the gene for the putative basic FGF receptor, can also be detected in the SFO. The results all support the possibility that this growth factor may modulate the known physiological functions of the SFO.