The study was undertaken to determine the antagonism of morphine analgesia by the standardized ginseng extract G115 from Panax ginseng, the inhibitory effects of orally administered G115 on the development of morphine-induced tolerance and physical dependence, the hepatic glutathione levels, the inhibitory effects of intraperitoneally administered G115 on the dopamine receptor supersensitivity, and the reverse tolerance to the locomotor accelerating effect of morphine. G115 significantly inhibits the development of morphine-induced tolerance and physical dependence, the hepatic glutathione level decrease induced by morphine multiple injections, the development of morphine-induced dopamine receptor supersensitivity, and reverse tolerance to the locomotor accelerating effect of morphine. It did not, however, antagonize morphine analgesia.