Growth of preantral follicles is mainly controled by gonadotropins. Activin stimulates synthesis of FSH at the pituitary gland, and on the other hand inhibin which is secreted by antral follicles is transferred to the pituitary gland, and the complex of inhibin and TGF β-receptor type III ihibites the function of Actibin receptors, resulting in a decrease of pituitary FSH secretion. In folliculiogenesis, a strong expression of activin mRNA is observed from the preantral follicle stage and Activin induces expression of FSH-R mRNA on granulosa cells of preantral follicles. Thereafter the follicles grow in a FSH-dependent manner. As the follicles grow larger, the activin-rich follicular environment changes to inhibin/follistatin-rich. The inhibin/follistatin-rich environment with decreases of FSH- and Activin-requirement on development of larger follicles would suppress development of smaller follicles in ovary through suppression of FSH secretion. Activin, inhibin and follistatin possibly play an important role in regulating physiolosical follicular development.