The physiologic role of corticotropin-releasing hormone (CRH) was examined in the ovary. We investigated the effects of CRH on steroidogenesis in rat and human granulosa cells in vitro as well as the direct effects of CRH on the ovary in vivo. We further examined the gene expression of CRH in human granulosa cells. CRH significantly inhibited the production of estradiol (E2) and progesterone (P4) in rat and human granulosa cells in vitro. These inhibitory effects were completely abolished by alpha-helical CRH, a CRH receptor antagonist. Forskolin-induced increase in E2 and P4 production was attenuated by CRH. On the other hand, CRH significantly increased serum concentrations of E2 and corticosterone in vivo in hypophysectomized rats, but this increase was completely blocked by adrenalectomy. It is probable that these effects did not result from a direct action on the ovary but were an indirect effect via the adrenal gland. Finally, by reverse transcriptase polymerase chain reaction we demonstrated that CRH mRNA was expressed in human granulosa cells. Our findings indicate that CRH exerts inhibitory effects on steroidogenesis in rat and human granulosa cells, acting through the CRH receptor. These effects are attributed to cellular events downstream of cyclic adenosine monophosphate generation. CRH seems to modulate steroidogenesis via autocrine or paracrine actions in the ovary.