Individual vulnerability to the reinforcing properties of drugs appears to be an essential characteristic predisposing humans to addiction. In animals, a greater behavioral reactivity to a mild stress, such as exposure to a novel environment, is an index of the vulnerability to acquire amphetamine self-administration. Biological responses to stress as well as behavioral reactivity may predict such a vulnerability. In the present study, rats with a longer duration of corticosterone secretion after exposure to novelty showed facilitation of acquisition of amphetamine self-administration. Furthermore, corticosterone administration in nonpredisposed individuals increased the reinforcing value of the drug and facilitated the acquisition of amphetamine self-administration. These results indicate that the stress-related activity of the hypothalamic-pituitary-adrenal axis may play a role in the pathogenesis of psychostimulant addiction.