Both serotonin-1B (5-HT1B) receptors and stress modulate the behavioral and neurobiological effects of psychostimulant drugs. In order to examine how these factors interact to influence the development of behaviors associated with addiction, we used viral-mediated gene transfer to transiently increase expression of 5-HT1B receptors in the nucleus accumbens (NAc) shell along with exposure to repeated mild stress (novelty + saline injection) in rats. Once the viral-mediated increases in gene expression had dissipated, the resulting effects of this 5-HT1B/stress pairing on the acute locomotor response to amphetamine and on the development of psychomotor sensitization were examined. We report that the increasing expression of 5-HT1B receptors on the terminals of NAc shell neurons that project to the ventral tegmental area and repeatedly exposing rats to mild stress subsequently enhance the acute locomotor-activating effects of amphetamine. In addition, the development of psychomotor sensitization (both locomotor activity and stereotypy components) is facilitated. These results suggest that serotonin signaling through NAc 5-HT1B heteroreceptors can interact with stress to increase susceptibility to the enduring forms of drug-induced plasticity that are associated with addiction.