Follicular production of prostaglandins (PGs) is essential for ovulation, but the factors mediating gonadotropin-induced secretion of PGE and PGF₂ remain largely unknown. We tested the hypothesis that gonadotropin-induced changes in progesterone and its receptor (PR) mediate the increase in periovulatory PGs. Heifers were treated with PGF₂ and GnRH to induce luteolysis and the LH/FSH surge (ovulation occurs 30 h post-GnRH). Since there are two increases in intrafollicular progesterone/PR mRNA during the bovine periovulatory period, we first examined the temporal pattern of PG production by follicles collected at 0, 3.5, 6, 12, 18, and 24 h post-GnRH. Although PGs did not increase in the follicular fluid until 24 h post-GnRH, acute secretion of PGs by follicle wall (theca + granulosa cells) was initiated by 18 h and had increased many-fold by 24 h post-GnRH. In vitro, FSH and LH induced dramatic transient increases in PG production by follicle wall and granulosa, but not theca, cells isolated from preovulatory follicles (0 h post-GnRH). PG accumulation peaked on day 2 of culture, mimicking the secretion pattern following a gonadotropin surge in vivo. In cultures of follicle wall and granulosa cells, the PR antagonist mifepristone (MIFE, 1 µM) inhibited LH-induced PG secretion and the progestin medroxyprogesterone acetate (MPA, 1 or 10 µM), but not the glucocorticoid dexamethasone (1 or 10 µM), overcame the effect of MIFE on PGs. Semi-quantitative RT-PCR revealed that MIFE inhibited LH-induced expression of COX-2 mRNA in granulosa cells in vitro. Again, treatment with MPA overcame the effect of MIFE. Together, these results provide strong evidence that periovulatory increases in COX-2 mRNA, PGE, and PGF₂ are mediated by gonadotropin-induced increases in progesterone/PR, indicating that in some species there is an important functional relationship between these pathways in the ovulatory cascade.