Adenosine deaminase is an enzyme of purine metabolism that has largely been considered to be cytosolic. A few years ago, adenosine deaminase was reported to appear on the surface of cells. Recently, it has been demonstrated that adenosine deaminase interacts with a type II membrane protein known as either CD26 or dipeptidylpeptidase IV. In this study, by immunoprecipitation and affinity chromatography it is shown that adenosine deaminase and A1 adenosine receptors interact in pig brain cortical membranes. This is the first report in brain demonstrating an interaction between a degradative ectoenzyme and the receptor whose ligand is the enzyme substrate. By means of this interaction adenosine deaminase leads to the appearance of the high-affinity site of the receptor, which corresponds to the receptor-G protein complex. Thus, it seems that adenosine deaminase is necessary for coupling A1 adenosine receptors to heterotrimeric G proteins.