Extracellular adenosine mediates a multitude of biological effects, including wakefulness, antiarrythmia, bronchoconstriction and response to ischemia and oxidative stress. A family of four G-protein coupled adrenoceptors, A1, A2A, A2B and A3, is responsible for these effects. A3, which couples to Gi/o, is expressed in a wide range of human tissues, but most predominantly in the lung and liver. Recent animal model studies have shown that A3 receptors play important roles in brain ischemia, immunosuppresion, and bronchospasm. A3 receptor agonists and/or agonists may have important clinical value in the trea™ent of asthma and inflammation. Mice lacking A3 receptors display reduced mast cell degranulation and bronchoconstriction in response to adenosine.