Four adenosine receptors have been cloned from many mammalian and some non-mammalian species. In each case the translated part of the receptor is encoded by two separate exons. Two separate promoters regulate the A1 receptor expression, and a similar situation may pertain also for the other receptors. The receptors are expressed in a cell and tissue specific manner, even though A1 and A2B receptors are found in many different cell types. Emerging data indicate that the receptor protein is targeted to specific parts of the cell. A1 and A3 receptors activate the Gi family of G proteins, whereas A2A and A2B receptors activate the Gs family. However, other G proteins can also be activated even though the physiological significance of this is unknown. Following the activation of G proteins several cellular effector pathways can be affected. Signaling via adenosine receptors is also known to interact in functionally important ways with signaling initiated via other receptors.