We have previously shown that the release of corticotrophin-releasing factor 41 (CRF-41) induced by a variety of neurotransmitters and depolarizing agents from the rat hypothalamus in vitro is inhibited by morphine. In order to further characterize the opiate receptors mediating this inhibitory action, we have now investigated the effects of a variety of opioid compounds with relatively high selectivity for µ-, ĸ- and δ-opiate receptors on K<sup>+</sup>-stimulated CRF-41 release. The selective µ-opioid receptor agonist 202–250 inhibited K+-evoked CRF-41 release in a dose-dependent manner with a maximum inhibition of approximately 60% at 10<sup>–5</sup> M (p < 0.01), as did the K-selective agonists PD-117,302 and U-50,488, with a similar plateau in response of approximately 40% inhibition at 10<sup>–6</sup> M (p < 0.05). The effects of these agonists were specifically reversed by the µ- and K-receptor antagonists naloxone and MR2266, repectively, while the specific δ-receptor antagonist ICI 154,129 was ineffective. Both naloxone and MR2266 slightly but significantly increased the basal release of CRF-41. The δ-agonist D-Pen<sup>2,5</sup>-enkephalin was without significant effect in the same dose range. These data suggest that both µ- and K-receptors, but not δ-receptors, mediate the inhibitory effect of opiates on stimulated CRF-41 release from the rat hypothalamus.