Follicular prostaglandin concentrations increase following the gonadotrophin surge in domestic animals and rodents approximately 10 h before follicle rupture, suggesting a unifying role for prostaglandins in the timing of ovulation. However, little is known about prostaglandin production by the primate ovulatory follicle. In this study, adult female macaques received gonadotrophins to promote follicular development. Granulosa cells, follicular fluid, and ovaries were collected before (0 h) and 12, 24 or 36 h after administration of the ovulatory stimulus, human chorionic gonadotrophin (HCG). Cyclooxygenase (COX) isoform expression was assessed by reverse transcription-polymerase chain reaction and immunocytochemistry and follicular prostaglandin production was determined by enzyme immunoassay. COX-2 mRNA expression in granulosa cells was low at 0 h, rose 50-fold by 12 h, and remained elevated through to 36 h. COX-2 immunostaining was present in granulosa cells after, but not before, exposure to HCG. COX-1 mRNA levels did not change during the periovulatory interval, and COX-1 immunostaining of granulosa cells was not detected. Follicular fluid prostaglandin (PG) E (2) and PGF (2alpha) concentrations were low through to 24 h but increased 100-fold at 36 h. The elevated follicular prostaglandin concentrations 4-16 h before the expected time of ovulation support the hypothesis that the time between the LH surge and increased follicular prostaglandins determines the length of the periovulatory period. Differences between the localization and timing of COX-2 expression in monkey versus non-primate follicles suggest that the pattern of COX-2 expression and activity has aspects unique to primates.