Mice carrying a null mutation for either of the two cyclooxygenase (COX) isoenzymes, necessary for prostanoid production, exhibit sev- eral isotype-specific reproductive abnormalities. Mice deficient in COX-1 are fertile but have decreased pup viability, whereas mice deficient in COX-2 fail to ovulate and have abnormal implantation and decidualization responses. The present study identifies the spe- cific contribution of each COX isoenzyme in hypothalamic, pituitary, and ovarian function and establishes the pathology and rescue of the anovulatory syndrome in the COX-2-deficient mouse. In both COX-1- and COX-2-deficient mice, pituitary gonadotropins were selectively increased, whereas hypothalamic LHRH and serum gonadotropin levels were similar to those in wild-type animals (1/1). No significant differences in serum estrogen or progesterone were noted among the three genotypes. Exogenous gonadotropin stimulation with PMSG and hCG produced a comparable 4-fold increase in ovarian PGE2 levels in wild-type and COX-12/2 mice. COX-22/2 mice had no in- crease in PGE2 over PMSG-stimulated levels. Wild-type and COX- 12/2 mice ovulated in response to PMSG/hCG; very few COX-22/2 animals responded to this regimen. The defect in ovulation in COX-2 mutants was attributed to both an abnormal cumulus oophorum expansion and subsequent stigmata formation. Gonadotropin stim- ulation and concurrent treatment with PGE2 or interleukin-1b re- sulted in ovulation of COX-22/2 mice comparable to that in COX-21/1, whereas treatment with PGF2a was less effective. Collectively, these data demonstrate that COX-2, but not COX-1, is required for the gonadotropin induction of ovarian PG levels; that COX-2-related prostanoids are required for stabilization of the cumulus oophorum during ovulation; and that ovulation can be restored in the COX-22/2 animals by simultaneous treatment with gonadotropins and PGE2 or interleukin-1b .( Endocrinology 140: 2685-2695, 1999)