The biological effects of estrogens are largely mediated through estrogen receptors (ERs), which belong to the nuclear receptor gene family of transcription factors. ER-alpha36 has been recently identified as a new variant of ERalpha, but its expression and roles in female reproduction system remain unknown. Immunocytochemistry and confocal microscopy were employed to observe ER-alpha36 distribution in mouse ovary during postnatal development and in oocyte during meiotic maturation. ER-alpha36 was consistently present in the nuclei of oocytes regardless of follicular growth stage and mouse age until germinal vesicle breakdown (GVBD). Its immunosignal was smeared in granulosa cells. However, the ER-alpha36 signal is up-regulated and found in cytoplasm with little or no nuclear staining during corpus luteum development. ER-alpha36 was also found in theca cells. We showed by Western blot that ER-alpha36 was expressed in mouse oocytes at various maturation stages. When the function of nuclear ER-alpha36 was blocked by microinjecting anti-ER-alpha36 specific antibody into the germinal vesicle (GV) of mouse oocytes, the first polar body emission occurred earlier in a higher proportion of oocytes compared to the control. These results suggest that ER-alpha36 may play critical roles in mouse ovarian folliculogenesis and oocyte development.