Preconditioning is a powerful form of (myocardial) protection that follows brief sublethal ischemia. G-protein-coupled receptors constitute the trigger for entrance to the preconditioned state. In conjunction with other receptors, various membrane adenosine receptors play an important role in the transduction of extracellular signals, leading to protection by preconditioning, lasting 1-3 hr. Adenosine A(1)- and A(3)-receptors mediate inhibition of adenylate cyclase via a guanine nucleotide binding inhibitory protein (G(i/o)). A(2)-receptors couple to a comparable stimulatory protein (G(s)). Adenosine receptors are especially abundant in the central nervous system; in lesser numbers, they are found in many tissues, including the heart. A(1)-receptors are located on cardiomyocytes and vascular smooth muscle cells, A(2)-receptors on endothelial and vascular smooth muscle cells, and A(3)-receptors on ventricular myocytes. Ischemic preconditioning by endogenous adenosine takes place through A(1)- and A(3)-receptors. A(2A/B)-receptor activation results in vasodilation. The relevance of cellular mediators, such as 5'-nucleotidase, to generate adenosine for preconditioning is controversial. In contrast, the role of protein kinase C (PKC) is clearly established. Signals from different receptors converge at PKC, reaching a threshold activation of the kinase necessary to induce protection. Tyrosine and mitogen-activated protein kinases may play a role in addition to PKC. The exact products downstream responsible for the memory of preconditioning are elusive. A prime candidate for the end-effector of preconditioning is the K(ATP) channel. Preconditioning with adenosine-receptor agonists offers the possibility for treatment of coronary artery disease, but research in this field is still in its infancy.