The analgesic action of beta-endorphin, as observed in the hot plate test with rats, was effectively suppressed by intracerebroventricular (i.c.v.) injection of caerulein and cholecystokinin octapeptide (CCK-8). The effect of caerulein was particularly striking; this peptide in doses of more than 0.09 nM lessened or abolished the analgesic effect of beta-endorphin in a dose of 0.7 nM. On the other hand, non sulfated CCK-8 had no significant effect on beta-endorphin-induced analgesia.