Effects of i.c.v. pretreatment with antisense oligodeoxynucleotides (antisense oligos) targeted against either mu- or delta-opioid receptors on morphine-induced release of growth hormone (GH) and prolactin were studied in male rats. The stimulation of GH secretion by i.c.v. morphine was completely inhibited by the antisense oligo targeted against the cloned mu-opioid receptor and significantly reduced by the antisense oligo targeted against the cloned delta-opioid receptor. The antisense oligo targeted against the cloned mu-opioid receptor, but not that targeted against the cloned delta-opioid receptor, abolished the stimulatory effect of acute morphine on prolactin secretion. It is concluded that both the GH and prolactin secretion stimulating effect of morphine is mainly mediated by the cloned mu-opioid receptor. Further, the cloned delta-opioid receptor is involved in the morphine-induced stimulation of GH secretion.