Two major subclasses of adenosine receptors have been distinguished in the central nervous system, termed A1 and A2. They are coupled to G-proteins and regulate the activity of adenylyl cyclase, potassium channels and several other effector systems. Autoradiographic studies have shown that A1 receptors are mainly found in the hippocampus and the cerebellum, whereas A2 receptors are almost exclusively located in the striatum and olfactory tubercle. Furthermore, a novel adenosine binding protein was identified in bovine striatum by radioligand binding with [3H]5'-N-ethylcarboxamidoadenosine ([3H]NECA). The pharmacological profile of this NECA binding protein has been determined in competition experiments with adenosine receptor ligands. It can be distinguished from that of A2 adenosine receptors and other adenosine binding proteins such as S-adenosylhomocysteine hydrolase and the adenosine transporter.