Objectives: Bok, Bcl-2-related ovarian killer, is a proapoptotic Bcl-2 family protein identified in the ovary based on its dimerization with the antiapoptotic protein Mcl-1. The present study examined the hormonal regulation and localization of Bok messenger RNA levels in the mouse ovary during the follicle development. Methods: The animals were implanted subcutaneously with Silastic brand capsules containing the synthetic estrogen, DES at $21{sim}23$ days of age. Ovaries were collected $1{sim}3$ days after implantation for RNA analysis and in situ hybridization. Some mice were removed capsule for $1{sim}2$ days to induce ovarian follicle apoptosis. Ovaries were also collected from 26 day-old immature mice at various times after treatment with 10 IU PMSG. Some mice received a single intraperitoneal injection of 10 IU hCG to induce ovulation, and ovaries were obtained at different time intervals for Northern blot and in situ hybridization analysis, respectively. Results: Treatment of immature mice with diethylstilbestrol (DES) for $24{sim}48$ h increased ovarian Bok mRNA levels. Bok mRNA was remained the same levels in mice removed DES for $24{sim}48$ h to induce apoptosis. High signals of Bok mRNA after DES treatment were detected in granulosa cells of early antral follicles. Treatment of immature mice with PMSG for 12 h increased markedly ovarian Bok mRNA expression which was detected mainly in preantral and atretic follicles. Interestingly, low levels of Bok mRNA were also expressed in granulosa cells of preovulatory follicles. Treatment of PMSGprimed mice with hCG stimulated strongly ovarian Bok mRNA expression at $6{sim}9$ h. At that time, Bok mRNA was expressed in granulosa cells of atretic and small growing follicles. Conclusion: These results demonstrate that Bok is one of proapoptotic Bcl-2 members expressed in early growing and atretic follicles during the ovarian follicular development. Gonadotropins induce a transient increase of Bok gene expression in granulosa cells of preantral and preovulatory follicles indicating some role in the ovulatory process.