The orphan nuclear receptors are a subset of the nuclear receptor family for which the ligands have not been defined. We have studied two of these, steroidogenic factor 1 (SF-1, NR5A1) and liver receptor homolog-1 (LRH-1, NR5A2). Both function as transcription factors in the ovary and other tissues and target the same DNA recognition sequence. LRH-1 is specific to the granulosa cells in the ovary, while SF-1 is found in granulosa, theca, and interstitial cells. Conditional depletion of LRH-1 in mice prior to the ovulatory signal via Cre/lox technology obviates ovulation. Global chromatin immunoprecipitation analysis demonstrated extensive reorganization of the LRH-1 cistrome in response the LH surge. After the LH surge, LRH-1 depletion results in impairment of luteal development and function, resulting in infertility. SF-1 depletion before ovulation disrupts follicle development, while depletion after the ovulatory signal compromises both ovulation and luteal function. Recent studies of the ovarian reserve indicate that LRH-1 expression is initiated in a subset of primordial follicles, poising them for entry into the developing pool, an event inhibited by conditional knockout. Downstream effectors include quiescence factors and genes associated with epithelial-mesenchymal transformation of granulosa cells. SF-1 is expressed in the prenatal ovary and depletion beginning prior to birth reduces the assembly of primordial follicles. There is also substantial inhibition of activation of those that develop. Among downstream effectors of SF-1 on the primordial pool are the mTor-Kit signaling system and forkhead transcription factor Foxl2. In spite of employing the same target motif, there seems to be little compensation by either orphan receptor when the other is depleted, either in the ovulatory or perinatal context. In summary, LRH-1 and SF-1 are essential regulators of multiple aspects of ovarian function, from follicle formation, to activation, development, ovulation and luteal function. Supported by CIHR project grant 166020.