Dynorphin (Dyn) A stimulates the release of ACTH in fetal sheep, a response that involves NMDA receptors but not the secretogogues CRH or AVP. We now find that neither Dyn A_1-13 (0.5 mg/kg, iv) nor NMDA (4 mg/kg, iv) elicits ACTH release in postnatal lambs. This led us to hypothesize that Dyn A_1-13 and NMDA might act to release placental ACTH. However, the ability of Dyn A_1-13, NMDA and the -opioid receptor agonist U50,488H (1 mg/kg, iv) to release ACTH was lost after either fetal hypophysectomy (HX, n=4) or hypothalamo-pituitary disconnection (HPD, n=4). These results indicate that neither the placenta nor fetal pituitary is the site of action for these agonists, and suggest a hypothalamic or supra-hypothalamic site of action. Furthermore, the release of ACTH by Dyn A_1-13 and NMDA were abolished after pretreatment with indomethacin, suggesting that Dyn A_1-13 and NMDA might cause the release of a prostanoid, possibly from the placenta, that subsequently acts at the hypothalamus or serves as a permissive factor in the action of Dyn A_1-13 and NMDA at the hypothalamus.