The antagonism against clonidine-induced analgesia by ginseng saponin (GS) and the inhibitory effect of GS on the development of clonidine-induced tolerance were evaluated in mice. GS, when administered systemically, intracerebrally and intrathecally, antagonized significantly the analgesic effect of clonidine. GS, when injected intraperitoneally not only inhibited the development of clonidine-induced analgesic tolerance, but also enhanced the analgesic effect of clonidine on the 2nd and 5th day. Naloxone did not antagonize the analgesic effect of clonidine and had no influence on the deveolpment of tolerance of both acute and delayed types. These results indicate that the antagonism against clonidine-induced analgesia and the inhibition of the deveolpment of clonidine-induced tolerance by GS are not mediated by the opioid mechanism.