lunar synchronization of in vitro steroidogenesis in ovaries of the golden rabbitfish siganus guttatus bloch to assess the relationship between lunar cycle and steroidogenesis in the ovaries of the golden rabbitfish siganus guttatus the intact follicles of oocytes were incubated in vitro with human chorionic gonadotropin hcg and seven steroid hormones 17alpha20betadihydroxy4pregnen3one dhp 17alpha20beta21trihydroxy4pregnen3one 20betas 17alphahydroxyprogesterone 17alphaohp progesterone p cortisol estradiol17beta e2 and testosterone during the two lunar phases the new moon 1 week before spawning and the first lunar quarter just before spawning around the new moon germinal vesicle breakdown gvbd could not be induced by addition of hcg or any steroid hormones around the first lunar quarter gvbd was induced by addition of hcg dhp 20betas 17alphaohp p and cortisol dhp was the most potent steroid hormone when the intact follicles of oocytes were incubated with hcg in both lunar phases the production of e2 and dhp measured by enzyme immunoassay decreased and increased significantly from the new moon to the first lunar quarter respectively these results suggest that the ovarian follicles produce e2 around the new moon and dhp around the first lunar quarter and that the productionconversion of the steroid hormones is under the influence of gonadotropins the synchronous increase in ovarian activity supports the hypothesis that lunar periodicity is a major factor for the ovarian development of s guttatus