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The influence of the endocannabinoid system on the sensitisation to the rewarding effects of morphine in the place conditioning paradigm was evaluated. In mice pretreated with morphine this drug induces place preference with lower doses. Pretreatment with non-rewarding doses of the cannabinoid agonist WIN 55,212-2 (0.5 and 1 mg/kg) induces sensitisation to the rewarding effects of morphine. However, the pretreatment with the cannabinoid antagonist SR 144716A plus morphine or WIN 55,212-2 blocks it. Our results suggest the existence of cross-talk between cannabinoids and opiates on the sensitisation to morphine and the implication of the endocannabinoid system in the process of sensitisation to opiates.