Increasing evidence has shown that excess androgen may be a main cause of polycystic ovary syndrome (PCOS). However, the molecular mechanism of androgen action on the ovary is unclear. To investigate the possible impacts of androgen on early follicular development, neonatal mouse ovaries mainly containing primordial follicles were cultured with testosterone. We demonstrated that the number of primary follicles was increased after 10 d culture with testosterone treatment via phosphatidylinositol 3-kinase/Akt pathway. Androgen induced Forkhead box (Foxo)-3a activation, and translocation of Foxo3a protein from oocyte nuclei to cytoplasm, which might be a key step for primordial follicle activation. Interestingly, testosterone was also capable of down-regulating growth and differentiation factor-9 expression via its receptor. In summary, we infer that intraovarian excess androgen in PCOS might result in excess early follicles by inducing oocyte Foxo3a translocation and follicular arrest by down-regulating growth and differentiation factor-9 expression.