A_2A adenosine receptors are highly expressed in the striatum where they modulate dopaminergic activity. The role of A_2A receptors in psychostimulant action is less well understood because of the lack of A_2A-selective compounds with access to the central nervous system. To investigate the A_2A adenosinergic regulation of psychostimulant responses, we examined the consequences of genetic deletion of A_2A receptors on psychostimulant-induced behavioral responses. The extent of dopaminergic innervation and expression of dopamine receptors in the striatum were indistinguishable between A_2A receptor knockout and wild-type mice. However, locomotor responses to amphetamine and cocaine were attenuated in A_2A knockout mice. In contrast, D₁-like receptor agonists SKF81297 and SKF38393 produced identical locomotor stimulation and grooming, respectively, in wild-type and A_2A knockout mice. Similarly, the D₂-like agonist quinpirole produced motor-depression and stereotypy that were indistinguishable between A_2A knockout and wild-type mice. Furthermore, attenuated amphetamine- (but not SKF81297-) induced locomotion was observed in pure 129-Steel as well as hybrid 129-Steel×C57BL/6 mice, confirming A_2A receptor deficiency (and not genetic background) as the cause of the blunted psychostimulant responses in A_2A knockout mice. These results demonstrate that A_2A receptor deficiency selectively attenuates psychostimulant-induced behavioral responses and support an important role for the A_2A receptor in modulating psychostimulant effects.