Behavioral and biochemical studies have suggested a functional link between the endogenous cannabinoid and opioid systems. Different hypotheses have been proposed to explain the interactions between opioid and cannabinoid systems such as a common pathway stimulating the dopaminergic system, a facilitation of signal-transduction- and/or a cannabinoid-induced enhancement of opioid peptide release. However, at this time, all the studies have been performed with exogenous agonists (delta-9-tetrahydrocannabinol or morphine), leading to a generally excessive stimulation of receptors normally stimulated by endogenous effectors (anandamide or opioid peptides) in various brain structures. To overcome this problem, we have measured various behavioral responses induced by the stimulation of the endogenous opioid system using the dual inhibitor of enkephalin-degrading enzymes, RB101, in CB1 receptor knockout mice. Thus, analgesia, locomotor activity, anxiety and antidepressant-like effects were measured after RB101 administration (80 and 120 mg/kg i.p. or 10 mg/kg, i.v.) in CB1 receptor knockout mice and their wild-type littermates. In all the experiments, inhibition of enkephalin catabolism produced similar modifications in behavior observed in CB1 knockout and wild-type mice. These results suggest limited physiological interaction between cannabinoid and opioid systems.