In rodents, drugs of abuse induce locomotor hyperactivity, and repeating injections enhances this response. This effect, called behavioral sensitization, persists many months after the last administration, thus mimicking long-term sensitivity to drugs observed in human addicts. We show here that, in naïve animals, noradrenergic and serotonergic systems, besides their behavioral activating effects, inhibit each other by means of the stimulation of alpha1b-adrenergic and 5-HT(2A) receptors and that this mutual inhibition vanishes with repeated injections of d-amphetamine; this uncoupling may be responsible for behavioral sensitization and for an increased reactivity of dopaminergic neurons. First, after repeated d-amphetamine injections, a d-amphetamine challenge induces a dramatic increase in cortical extracellular norepinephrine (NE) levels. This increased cortical NE release still occurs after 1 month of withdrawal but is diminished or blocked if sensitization is performed in the presence of prazosin, SR46349B, or both alpha1-adrenergic and 5-HT(2A) receptor antagonists, respectively. A strong correlation between increases in cortical extracellular NE levels and the expression of behavioral sensitization was found. Second, repeated d-amphetamine injections induce an increased reactivity of serotonergic neurons measured by cortical extracellular serotonin (5-HT) levels after the administration of a 5-HT releaser, p-chloroamphetamine. Third, knockout mice for alpha1b-adrenergic (alpha1b-AR KO) or 5-HT(2A) (5-HT(2A)-R KO) receptor, respectively, exhibit a behavioral and biochemical hyperreactivity to the acute injection of p-chloroamphetamine (alpha1b-AR KO; 5-HT levels) and d-amphetamine (5-HT(2A)-R KO; NE levels). Uncoupling between noradrenergic and serotonergic neurons may occur not only in addiction but also during chronic stressful situations, thus facilitating the onset of mental illness.