Antagonistic and reciprocal interactions are known to exist between adenosine and dopamine receptors in the striatum. In the present study, double immunofluorescence experiments with confocal laser microscopy showed a high degree of colocalization of adenosine A2A receptors (A2AR) and dopamine D2 receptors (D2R) in cell membranes of SH-SY5Y human neuroblastoma cells stably transfected with human D2R and in cultured striatal cells. A2AR/D2R heteromeric complexes were demonstrated in coimmunoprecipitation experiments in membrane preparations from D2R-transfected SH-SY5Y cells, and from mouse fibroblast Ltk- cells stably transfected with human D2R (long form) and transiently cotransfected with the A2AR double-tagged with hemagglutinin. Long term exposure to A2AR and D2R agonists in D2R-cotransfected SH-SY5Y cells resulted in coaggregation, cointernalization and codesensitization of A2AR and D2R. These results give a molecular basis for adenosine-dopamine antagonism at the membrane level and have implications for treatment of Parkinson's disease and schizophrenia, in which D2R are involved.