Maturation of the fetal hypothalamus-pituitary-adrenal (HPA) axis is critical for the timely somatic development of the fetus and readiness for birth. Recently, we proposed that prostaglandin generation within the fetal central nervous system is critical for the modulation of hypotension-induced fetal ACTH secretion. The present study was designed to test the hypothesis that the preparturient increase in fetal ACTH secretion is dependent upon fetal CNS prostaglandin synthesis mediated by the activity of Prostaglandin Endoperoxide Synthase-2 (PGHS-2, Cyclooxygenase-2, COX-2) in the fetal brain. We performed two studies in chronically-catheterized fetal sheep. In the first study, we infused nimesulide or vehicle intracerebroventricularly (icv) into singleton fetal sheep and collected blood samples until spontaneous parturition. Nimesulide significantly delayed parturition and inhibited fetal ACTH and POMC secretion, but did not prevent the preparturient rise in fetal plasma cortisol concentration. In the second study, we used twin fetuses. One fetus received icv nimesulide and the other icv vehicle. Nimesulide reduced brain tissue concentrations of PGE2, while not affecting plasma PGE2 concentrations, demonstrating an action restricted to the fetal brain. Nimesulide reduced PGHS-2 mRNA and increased PGHS-2 protein while not altering PGHS-1 mRNA or protein in most brain regions, suggesting an effect of the inhibitor on PGHS-2 turnover and relative specificity for PGHS-2 in vivo. We conclude that the preparturient increase in fetal ACTH and POMC are dependent upon the activity of PGHS-2 in the fetal brain. We also conclude, however, that the timing of parturition is not solely dependent upon ACTH in this species.