The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse perturb the normal functioning of this pathway. The nucleus accumbens (NAc) is a major terminal field of the mesolimbic dopamine neurons and modifications in neuronal structure and function in NAc accompany repeated exposure to psychomotor stimulants and other addictive drugs. Glutamatergic afferents to the NAc are thought to be crucial to the development of several aspects of addictive behavior, including behavioral sensitization and relapse to cocaine self-administration. Here we examine glutamatergic neurotransmission and synaptic plasticity in NAc neurons in vitro before and after repeated amphetamine treatment in vivo. We find that dopamine attenuates the response of NAc neurons to repetitive activation of glutamatergic afferents and thereby blocks long-term potentiation (LTP) induced by high-frequency afferent stimulation. Dopamine's effects are mimicked by dopamine receptor agonists and by amphetamine. In a second set of experiments, animals were treated with amphetamine daily for 6 days and brain slices were prepared after 8–10 days of withdrawal. In these slices, LTP in the NAc appears normal. However, acute exposure of such slices to amphetamine no longer modulates synaptic transmission or LTP induction. Thus, repeated exposure to amphetamine produces long-lasting changes in the modulation of glutamatergic synaptic transmission by amphetamine in the NAc. Our results support the notion that after psychostimulant exposure, excitatory synapses on NAc neurons alter their response to further psychostimulant for long periods of time. Synapse 51:1–10, 2004. © 2003 Wiley-Liss, Inc.