This work was designed to examine whether brain endomorphins (EM1 and EM2), the endogenous mu-opioid ligands, are involved in electroacupuncture (EA)-induced analgesia in the mice. C57BL/6J mice were given EA for 30 min and the effect of EA-induced analgesia was assessed by radiant heat tail flick latency (TFL). Intracerebroventricular (i.c.v.) injection of mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Tyr-Orn-Thr-Pen-Thr-NH(2) (CTOP), or antiserum against EM1 or EM2 was performed to see whether EA analgesia could be blocked. The results showed that: (1) i.c.v. injection of CTOP at 25-100 ng dose-dependently antagonized the analgesia induced by EA of 2 Hz, but not 100 Hz. (2) Intracerebroventricular injection of EM1 antiserum (5 ml, 1:1 or 1:10 dilution) dose-dependently antagonized 2 Hz, but not 100 Hz EA analgesia. (3) EM2 antiserum showed similar effect at 1:1 dilution. The results are interpreted to mean that endogenously released EM1 and EM2 and the cerebral mu-receptors are involved in mediating 2 Hz but not 100 Hz EA analgesia in the mice.