The steroidogenic pathway within the ovary gives rise to progestins, androgens and oestrogens, all of which act via specific nuclear receptors to regulate reproductive function and maintain fertility. The precise role of oestrogen in the ovary remains to be elucidated, hence the data presented here which arises from studies designed to resolve this issue. Oestrogens signal via two receptor subtypes ERalpha and ERbeta, both of which are present in the ovary. ERbeta, the most abundant mRNA, is primarily expressed by GC where it transduces signals from ovarian-derived and exogenous oestrogens. Specific roles for each of the ERs in the ovary have yet to be established, despite ER knockout studies indicating both are required for normal function. The ArKO mouse is a model of oestrogen insufficiency. These mice are infertile as a result of arrested folliculogenesis (at the antral stage) and a failure to ovulate. Trans/re-differentiation of somatic cells in the ovary gives rise to Sertoli cell-like and Leydig cell-like cells within abnormal follicular structures. Disruption to the balance of sex steroids in the ovary is likely to facilitate this phenotype. Future studies will focus on the regulation of somatic cell differentiation, assigning roles to individual ERs and establishing definitive targets of oestrogen action in the ovary.