The effects of intracerebroventricular (i.c.v.) administration of D-Phe-Cys-Tyr-D-Try-Orn-Thr-Pen-Thr-NH2 (CTOP), a selective mu-opioid receptor antagonist, (Allyl)2-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174864) and (N,N-Bisallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH (ICI 154129), selective delta-opioid receptor antagonists on blocking analgesia induced by beta-endorphin, morphine, D-Ala2-NMePhe4-Gly-ol-enkephalin (DAMGO), D-Ala2-D-Leu5-enkephalin (DADLE) and D-Pen2-enkephalin (DPDPE) administered i.c.v. were studied in male ICR mice. The analgesia was assessed by the tail-flick and paw-licking (hot-plate) tests. The potencies of opioid agonists injected i.c.v. for producing analgesia were DAMGO greater than DADLE greater than beta-endorphin greater than morphine greater than DPDPE. Intracerebroventricular administration of CTOP (0.05 micrograms) selectively antagonized inhibition of the tail-flick and paw-licking response induced by morphine, DAMGO or DADLE but not beta-endorphin or DPDPE. ICI 174864 (5 micrograms) and ICI 154129 (5 micrograms) injected i.c.v. selectively antagonized analgesia induced by DPDPE or DADLE but not beta-endorphin, morphine or DAMGO injected i.c.v. These results indicate that analgesia induced by morphine and DAMGO is mediated by the stimulation of mu-opioid receptors while analgesia induced by DPDPE is mediated by the stimulation of delta-opioid receptors. DADLE-induced analgesia is mediated by the stimulation of both mu- and delta-opioid receptors. Analgesia induced by beta-endorphin is mediated by neither mu- nor delta-opioid receptors.