Neurotrophin-4 (NT-4), a recently discovered novel member of the family of neurotrophic factors structurally related to nerve growth factor (NGF), is abundantly expressed in the Xenopus laevis ovary. In this study we have localized NT-4 mRNA expressing cells in the Xenopus ovary by in situ hybridization and have used this technique together with Northern blot analyses to quantify NT-4 mRNA expression during oogenesis in Xenopus. In situ hybridization of sections through the Xenopus ovary using an alpha-[35S]-dATP labeled Xenopus NT-4 mRNA specific probe showed an intense labeling over the cytoplasm of oocytes with a diameter of 50-200 microns corresponding to stage I according to Dumont (1972). Labeling was also seen over the cytoplasm of stages II to IV although with a lower intensity than over stage I oocytes. No labeling was seen over more mature oocytes of stages V and VI. NT-4 mRNA could not be detected in the early embryo from the onset of cleavage division to the neurula stage suggesting that the NT-4 gene is not expressed during Xenopus early embryogenesis. The confinement of NT-4 mRNA in the Xenopus ovary to immature oocytes suggests that NT-4 mRNA expression is strictly regulated during oogenesis and that the NT-4 protein could play a role as a maturation factor for immature oocytes.