It is well understood that several hormones, growth factors and proteases regurate the development of ovarian follicles, however, the mechanisms of the selection of dominant follicles has not been elucidated. Insulin-like growth factor (IGF)-binding proteins (IGF-BPs) are also involved in the regulation of follicular growth by controling the availability of IGF. One of the structural features IGF-BPs possess is thyroglobulin type-1 domain (Tg-1) on their C-terminal region. Although Tg-1 has been shown to function as a protease inhibitor, the role of it in gonadal tissues is unknown. In this study, we examined the possibility that Tg-1 acts as as a local modulator of folliculogenesis. Recombinant rat IGF-BP 1 and 5 as well as Tg-1 of rat thyroglobulin were expressed in E. coli. To evaluate the effect of Tg-1, chimeric proteins of IGF-BP 1 and 5 were constructed and also expressed. Primary follicles were enzymatically isolated from ovaries obtained from immature 14-day-old rats. One primary follicle was plated onto each well and cultured for several days with recombinant proteins. Diameter of follicles were measured through the culture period. Without recombinant proteins, the follicles gradually increased their size to formed antral-like structure, which was characteristic of the secondary follicular stage. IGF-BP 1 slightly inhibited the FSH-induces growth of primary follicles, whereas IGF-BP 5 showed the LH-like activity which induced a follicle to the stage of ovulation. The effects of Tg-1 on the follicles were similar to that of IGFBP-1. However, the phosphorylation of Akt in the follicles was stimulated by Tg-1 but not by IGF-BP 1. When two primary follicles were co-cultured in the same well with recombinant proteins, communication between those two follicles in the course of growth was observed. In conclusion, we found a novel function of thyroglobulin type-1 domain of IGF-BPs on the regulation of folliculogenesis.