In order to investigate the action of leptin on early follicular growth, preantral follicles, 95–115 μm in diameter were mechanically isolated from the ovaries of BDF1 hybrid immature (11-day-old) and adult (8-wk-old) mice, and cultured for 4 days in vitro. Follicular growth was assessed by daily changes in follicular diameter and by the amount of estradiol and immunoreactive (IR)-inhibin released into the culture medium at Day 4. Preantral follicles from immature mice showed a significant development in follicular growth as a result of stimulation by GH (1 mIU/ml), insulin-like growth factor (IGF)-I (100 ng/ml) FSH (100 mIU/ml), and GH (1 mIU/ml) FSH (100 mIU/ml). Although leptin at concentrations of 1–1000 ng/ml did not have any significant effect on follicular growth stimulated by IGF-I or GH, it significantly inhibited follicular growth in a dose-related manner when follicles were stimulated by IGF-I FSH and GH FSH, respectively, suggesting that leptin attenuated the additive effect of FSH. On the other hand, preantral follicles from adult mice were cultured in the presence of FSH, and FSH-dependent follicular growth was inhibited by leptin in a dose-related manner. Because FSH stimulates cAMP production, we investigated the involvement of cAMP in the inhibitory mechanisms of leptin. Preantral follicles from immature and adult mice were cultured in the presence of either 8-Br-cAMP or forskolin. Both 8-Br-cAMP and forskolin significantly increased follicular diameter and hormone secretion in both immature and adult mice. However, 8-Br-cAMP and forskolin-stimulated follicle growth and hormone secretion were significantly inhibited in immature mice by coadministration of leptin, whereas growth of preantral follicles from adult mice was not inhibited by addition of leptin to cultures. These results indicate that leptin causes an inhibitory effect on the early follicular development of both immature and adult mice, but the inhibitory mechanisms of leptin are different.