Prostaglandin E2 (PGE2) has been identified as a PG necessary for ovulation, but the ovulatory gonadotropin surge also increases PGF2 alpha levels in primate periovulatory follicles. To better understand the role of PGF2 alpha in ovulation, pathways utilized for PGF2 alpha synthesis by the primate follicle were examined. Monkeys were treated with gonadotropins to stimulate multiple follicular development; follicular aspirates and whole ovaries were removed before and at specific times after administration of an ovulatory dose of hCG to span the 40 h periovulatory interval. Human granulosa cells were also obtained (typically 34-36 h after hCG) from in vitro fertilization patients. PGF2 alpha can be synthesized from PGH2 via the aldo-keto reductase (AKR) 1C3. AKR1C3 mRNA and protein levels in monkey granulosa cells were low before hCG and peaked 24-36 h after hCG administration. Human granulosa cells converted PGD2 into 11 beta-PGF2 alpha, confirming that these cells possess AKR1C3 activity. PGF2 alpha can also be synthesized from PGE2 via the enzymes AKR1C1 and AKR1C2. Monkey granulosa cell levels of AKR1C1/AKR1C2 mRNA was low 0-12 h, peaked at 24 h, and returned to low levels by 36 h after hCG administration. Human granulosa cell conversion of [(3)H]PGE2 into [(3)H]PGF2 alpha was reduced by an AKR1C2-selective inhibitor, supporting the concept that granulosa cells preferentially express AKR1C2 over AKR1C1. In summary, the ovulatory gonadotropin surge increases granulosa cell expression of AKR1C1/AKR1C2 and AKR1C3. Both of these enzyme activities are present in periovulatory granulosa cells. These data support the concept that follicular PGF2 alpha can be synthesized via two pathways during the periovulatory interval.