Morphine (10 and 20 mg/kg, s.c.) does not modify dopamine release in the nucleus accumbens of cannabinoid CB(1) knock-out mice under conditions where it dose-dependently stimulates the release of dopamine in the corresponding wild-type mice. These results demonstrate that cannabinoid CB(1) receptors, regulate mesolimbic dopaminergic transmission in brain areas known to be involved in the reinforcing effects of morphine.