The present study examines the effect of opiates on α-MSH and β-endorphin release from perifused neurointermediate rat pituitaries, as stimulated by various secretagogues for which Ca ions and/or cAMP serve as messengers. α-MSH release stimulated by high K+ concentrations (5-min pulses) and veratridine depolarization, which is closely dependent on Ca2+ ñuxes, was abolished by both Leu-enkephalin and β-endorphin. A dose-response relationship between inhibition of α-MSH secretion and the concentration of Leu-enkephalin, with EDso – 10–9M, was observed. High K+-in-duced release of β-endorphin was likewise blunted by Leu-enkephalin. The stimulatory effect of the Ca2+ ionophore A 23187 was inhibited in a similar way as was that of CRF, which requires both Ca2+ fluxes and cAMP formation. The antagonist naloxone not only reversed the action of opiates, but also enhanced spontaneous hormonal output. In contrast, the effects of /-isoproterenol and forskolin, for which cAMP serves as a primary messenger, were unaffected in the absence of extracellular Ca ions and, also, in the presence of Leu-enkephalin. We conclude that opioid peptides may exert a direct inhibitory influence on the release of both α-MSH and β-endorphin and do so by interfering with the Ca2+ messenger system. In addition, these data also suggest the existence of an opiate-opiate negative feedba