Repeated treatment with the psychostimulant amphetamine produces behavioral sensitization that may represent the neural adaptations underlying some features of psychosis and addiction in humans. In the present study we investigated the role of adenosine A 2A receptors in psychostimulant-induced locomotor sensitization using an A 2A receptor knockout (A 2A KO) model. Daily treatment with amphetamine for 1 week resulted in an enhanced motor response on day 8 (by two-fold compared to that on day 1), and remained enhanced at day 24 upon rechallenge with amphetamine. By contrast, locomotor sensitization to daily amphetamine did not develop in A 2A KO mice on day 8 or 24, and this absence was not the result of a nonspecific threshold effect. The absence of behavioral sensitization was selective for amphetamine since daily treatment with the D 1 agonist SKF81297 (2.5 mg/kg) or the D 2 agonist quinpirole (1.0 mg/kg) produced similar behavioral sensitization in both WT and A 2A KO mice. Furthermore, coinjection of SKF81297 and quinpirole also resulted in indistinguishable locomotor sensitization in A 2A KO and WT mice, suggesting normal D 1 and D 2 receptor responsiveness. Finally, at the cellular level A 2A receptor inactivation abolished the increase in striatal dynorphin mRNA induced by repeated amphetamine administration. The selective absence of amphetamine-induced behavioral sensitization in A 2A KO mice suggests a critical role of the A 2A receptor in the development of psychostimulant-induced behavioral sensitization, and supports the pharmacological potential of A 2A adenosinergic agents to modulate adaptive responses to repeated psychostimulant exposure.