During mammalian oestrous cycles, ovarian follicles support the development and release of oocytes. But this is not just one-way support, as oocytes also contribute to follicle development. On p. 199, Liu and colleagues report that oocyte-specific expression of the transcription factor Foxo3a negatively regulates oocyte growth and follicular development. The researchersÕ previous work had suggested that the suppression of Foxo3a in oocytes Ð through activation of the phosphatidylinositol 3-kinase pathway Ð might be needed for follicular development and oocyte growth. To test this idea, the researchers generated transgenic mice that express constitutively active Foxo3a in their oocytes. The female transgenic mice, they report, are infertile because of retarded oocyte growth and follicular development, and anovulation. They also show that constitutively active Foxo3a causes reduced oocyte-specific expression of proteins that are required for follicle development. Overall, the researchers conclude that Foxo3a is an important intra-oocyte signalling molecule and suggest that their results might provide clues to the causes of premature ovarian failure in humans.