Locomotor activation produced by the indirect dopamine (DA) agonist amphetamine is reversed by the opiate-receptor antagonist naloxone. Since amphetamine-stimulated locomotion results from the release of DA within the nucleus accumbens (N.Acc.), it is possible that these effects of naloxone result either from a decrease in the pre-synaptic release of DA within the N.Acc. or from a disruption of the effects of DA at, or distal to, the post-synaptic DA receptor. In the present study, we investigated the effects of naloxone on the locomotor-activating properties of dopamine injected directly into the nucleus accumbens. Naloxone (0-2 mg/kg) had no significant effect of DA-stimulated locomotion; the lowest dose of naloxone tested (0.5 mg/kg) was shown to significantly disrupt the locomotor activation produced by amphetamine (0.5 mg/kg). In separate animals, very high doses of naloxone (5.0 mg/kg) had no significant effect on locomotor activation produced by the DA receptor agonist apomorphine in rats following 6-hydroxydopamine (6OHDA) denervation of the N.Acc. These results indicate that naloxone must disrupt amphetamine-stimulated locomotion through its action presynaptic to N.Acc. DA receptors.