Cortisol administered at a dose of 25 mg/kg 24 h before measurements decreased the prolactin secretion induced by intraventricularly given opioids (dynorphin, beta-endorphin, Met-enkephalin or D-Met-Pro-enkephalin-amide). The effect of cortisol was depressed by actinomycin D pretreatment. The cortisol-induced inhibition of the action of morphine was facilitated in adrenalectomized animals; measuring the effects of increasing doses of cortisol a maximal inhibition was obtained at a dose of 5 mg/kg. The opioid-induced corticosterone secretion was not affected 24 h after a single administration of cortisol. The present results show that the cortisol-induced inhibition of opioid-induced prolactin secretion is dependent on protein synthesis and independent of changes in drug metabolism, and of the type of opiate receptor preferentially affected by the opiate agonists employed.