Addiction is a chronic relapsing disorder hypothesized to be produced by drug-induced plasticity that renders individuals vulnerable to craving-inducing stimuli such as re-exposure to the drug of abuse. Drug-induced plasticity that may result in the addiction phenotype includes increased excitatory signaling within corticostriatal pathways which correlates with craving in humans and is necessary for reinstatement in rodents. Reduced cystine-glutamate exchange by system xc-appears to contribute to heightened excitatory signaling within the striatum, thereby posing this as a novel target in the treatment of addiction. In the present report, we examined the impact of repeated N-acetyl cysteine, which is commonly used to activate cystine-glutamate exchange, on reinstatement in rodents in a preclinical study and on craving in cocaine-dependent humans in a preliminary, proof of concept clinical experiment. Interestingly, repeated administration (seven days) of N-acetyl cysteine (60 mg/kg, IP) produced a significant reduction in cocaine- (10 mg/kg, IP) induced reinstatement, even though rats (N=10-12/group) were tested 24 hrs after the last administration of N-acetyl cysteine. The reduction in behavior despite the absence of the N-acetyl cysteine indicates that repeated N-acetyl cysteine may have altered drug-induced plasticity that underlies drug-seeking behavior. In parallel, our preliminary clinical data indicate that repeated administration (four days) of N-acetyl cysteine (1200-2400 mg/day) to cocaine-dependent human subjects (N=4/group) produced a significant reduction in craving following an experimenter-delivered IV injection of cocaine (20 mg/70 kg/60 sec). Collectively, these data demonstrate that N-acetyl cysteine diminishes the motivational qualities of a cocaine challenge injection possibly by altering pathogenic drug-induced plasticity.