It is well known that the nucleoside adenosine exerts a modulatory influence in the central nervous system by activating G protein coupled receptors. Adenosine A_2A receptors, the subject of the present review, are predominantly expressed in striatum, the major area of the basal ganglia. Activation of A_2A receptors interferes with effects mediated by most of the principal neurotransmitters in striatum. In particular, the inhibitory interactions between adenosine acting on A_2A receptors and dopamine acting on D₂ receptors have been well examined and there is much evidence that A_2A receptors may be a possible target for future development of drugs for treatment of Parkinson's disease, schizophrenia and affective disorders. Our understanding of the role of striatal A_2A receptors has increased dramatically over the last few years. New selective antibodies, antagonist radioligands and optimized in situ hybridization protocols have provided detailed information on the distribution of A_2A receptors in rodent as well as primate striatum. Studies on the involvement of A_2A receptors in the regulation of DARPP-32 and the expression of immediate early genes, such as nerve growth factor-induced clone A and c-fos, have pointed out an important role for these receptors in regulating striatopallidal neurotransmission. Moreover, by using novel selective antagonists for A_2A receptors and transgenic mice lacking functional A_2A receptors, crucial information on the behavioral role of striatal A_2A receptors has been provided, especially concerning their involvement in the stimulatory action of caffeine and the anti-Parkinsonian properties of A_2A receptor antagonists. In the present review, current knowledge on the distribution, biochemistry and function of striatal A_2A receptors is summarized.