The SULTlEl‐encoded estrogen sulfo‐transferase (EST) catalyzes sulfation of estrogen, resulting in its inactivation. Reduced fertility observed in SULT1E1 knockout (KO) female mice has previously been attributed to the deleterious effect of chronic exposure to high levels of circulating estrogen on placental function. We herein suggest that, in addition to placental dysfunction, this phenotype demonstrates that an excess of estrogen impairs ovulation. The role of SULT1E1 in ovulation is suggested by the substantially low ovulatory response in hCG‐treated SULT1E1 KO mice;a similar effect was observed when 17β‐estradiol was administered to wild‐type (WT) females. The normal rate of ovulation in SULT1E1 KO females may be restored by PGE2. Along this line, ovaries of human Chorionic Gonadotropin (hCG)‐treated SULT1E1 KO mice expressed low levels of cyclooxy‐genase‐2 (COX‐2) and its downstream TSG6;moreover, their ovaries contained a reduced number of expanded cumuli. Our results demonstrate, for the first time, that estrogen inactivation may allow the expression of COX‐2 and subsequent cumulus expansion, enabling normal ovulation. Our findings may be applied to novel treatments of human ovulatory failure.—Gershon, E., Hourvitz, A., Reikhav, S., Maman, E., Dekel, N. Low expression of COX‐2, reduced cumulus expansion, and impaired ovulation in SULT1E1‐deficient mice. FASEB J. 21, 1893–1901 (2007)