ABSTRACT The involvement of μ- and κ-opiate receptors in the regulation of LH and prolactin secretion was investigated in long-term ovariectomized rats using selective opiate receptor agonists and antagonists. The μ-agonists morphine and [d-Ala²,MePhe⁴,Gly⁵-ol]-enkephalin (DAGO) suppressed LH levels in a dose-related manner. The benzomorphane (−)-5,9-dimethyl-2′-hydroxy-2-(tetrahydrofurfuryl)-6,7-benzomorphan tartrate (MR 2034; a designated κ-agonist) also suppressed LH levels, whereas another benzomorphane κ-agonist (−)-5,9-dimethyl-2′-hydroxy-2-(2-methoxy-propyl)-6,7-benzomorphan hydrobromide (MRZ 2549) had no effect on the levels of this hormone. Pretreatment with the highly selective μ-antagonist β-funaltrexamine (β-FNA), the fumarate methyl ester derivative of naltrexone, blocked the actions of both μ-agonists and MR 2034, indicating that opiate suppression of LH secretion is mediated by μ-receptors. This was further confirmed by in-vitro studies: the KCl-induced release of LHRH from perifused hypothalami obtained from ovariectomized rats was significantly reduced by DAGO but not by MRZ 2549. Prolactin secretion was stimulated in a dose-dependent manner by both μ- and κ-agonists. The stimulation caused by morphine and DAGO was antagonized by β-FNA, whereas that caused by the κ-agonists MR 2034 and MZR 2549 was resistant to blockade by β-FNA but not by naloxone (an antagonist which blocks all classes of opiate receptors when given in high doses). Thus prolactin secretion seems to be regulated by both μ- and κ-opiate receptors, whereas the effects on LH secretion seem to involve μ-receptors only. J. Endocr. (1987) 114, 469–476