The effects of the novel kappa-opioid receptor agonist TRK-820 on the rewarding and locomotor-enhancing effects of morphine were investigated in mice. Morphine (1-5 mg/kg, s.c.) caused a dose-related preference for the drug-associated place. In contrast, TRK-820 (0.003-0.03 mg/kg, s.c.) did not produce a significant preference for either compartment of the test box. In combination studies, co-injection of TRK-820 (0.01 and 0.03 mg/kg, s.c.) with morphine significantly suppressed the morphine (5 mg/kg, s.c.)-induced place preference, and this effect of TRK-820 was antagonized by pretreatment with nor-BN1 (3 mg/kg, s.c.), a selective kappa-opioid receptor antagonist. TRK-820 also suppressed morphine-induced hyperlocomotion, and this suppression was also blocked by nor-BNI. These results suggest that TRK-820 suppresses the rewarding and locomotor-enhancing effects of morphine through the activation of kappa-opioid receptors. Thus, we propose that TRK-820 may be useful for controlling pain while reducing undesirable side-effects.