Neurotensin is an endogenous neuropeptide with neuronal perikarya or fibers distributed in the vicinity of the mesolimbic dopamine system. This observation, plus behavioral data showing that neurotensin injection into the nucleus accumbens blocks some behavioral effects of amphetamine, indicates that neurotensin may modulate the mesolimbic dopamine system. In this study it was shown that neurotensin given into the nucleus accumbens produces a dose-dependent blockade of locomotion and rearing initiated by dopamine injection into the nucleus accumbens. This effect is not mimicked by inactive neurotensin analogue nor some other endogenous neuropeptides. Since dopamine acts on postsynaptic dopamine receptors in the nucleus accumbens, neurotensin is acting, not on dopamine terminals, but on neurons or neuronal systems which are modulated by the mesolimbic dopamine system. This conclusion is supported by the facts that intra-accumbens injection of neurotensin does not alter accumbens levels of dopamine or its metabolites, nor does it affect the increase in dopamine metabolites produced by injection of neurotensin into the ventral tegmental area. Further, neurotensin was also found to block the dopamine-independent increase in locomotion and rearing produced by the injection of D-Ala2-Met- 5enkephalinamide into the nucleus accumbens. These data indicate that neurotensin acts on neurons in the nucleus accumbens to counteract the motor stimulant effects of dopamine or enkephalin. Therefore, in the nucleus accumbens, neurotensin is not acting to modulate the mesolimbic dopamine system, but rather appears to antagonize behavioral hyperactivity, regardless of the neurochemical initiation.