A single or repeated administration of morphine in mice produced hyperactivity, conditioned place preference (CPP) and postsynaptic dopamine (DA) receptor supersensitivity. The hyperactivity induced by morphine was evidenced by measuring the enhanced ambulatory activity using a tilting-type ambulometer. CPP effects were evaluated assessing the increased time spent by the mice to morphine and the inhibition of CPP by the decreased time spent by the mice in the white compartment. Postsynaptic DA receptor supersensitivity in mice displaying a morphine-induced CPP was evidenced by the enhanced response in ambulatory activity to the DA agonist, apomorphine (2 mg/kg, s.c.). The intraperitoneal injection of ginseng total saponin (GTS) from the root of Panax ginseng C.A. Meyer (Araliaceae), prior to and during the morphine treatment in mice inhibited morphine-induced hyperactivity and CPP. GTS inhibited the development of postsynaptic DA receptor supersensitivity. A single dose administration of GTS also inhibited apomorphine-induced climbing behavior, showing the antidopaminergic action of GTS at the postsynaptic DA receptor. These results suggest that the development of morphine-induced CPP may be associated with the enhanced DA receptor sensitivity and that GTS inhibition of the morphine-induced hyperactivity and CPP may be closely related with the inhibition of dopaminergic activation induced by morphine.