Repeated administration of addictive drugs induces neuronal apoptosis and the underlying mechanisms are not clear. Our present study investigated the effects of treatments with different addictive drugs on gene expression of murine double minute clone 2 (MDM2), a key negative regulator of p53 and an important mediator in cell apoptosis. The level of MDM2 gene expression in rat brain was assessed using in situ hybridization histochemistry. In normal adult rat brain, MDM2 expression was at a very low level but MDM2 mRNA-positive cells were detected in various regions including cortex, hippocampus, thalamus, amygdala, periaqueductal gray and locus ceruleus. After a single morphine injection, MDM2 gene expression increased significantly in hippocampus, amygdala and cortex; however, such up-regulation of MDM2 gene expression was significantly reduced after repeated morphine administration. Moreover, 24 h after cessation of chronic morphine exposure, MDM2 mRNA increased again to a level comparable to that of the acute morphine group. Acute heroin or cocaine administration also significantly increased MDM2 gene expression in hippocampus, but not in cortex. In thalamus, no change was detected after acute or chronic treatment with morphine, heroin, or cocaine. Thus we demonstrated for the first time that the administration of addictive drugs regulate MDM2 gene expression in distinct rat brain regions and these data suggest that MDM2 may play an important role in the development of drug addiction.