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Extracellular adenine and uracil 5'-nucleotides are important signalling molecules that exert a great variety of effects in numerous tissues and cell types through the activation of P2 receptors. In the past eight years, an extended series of P2 receptors (P2X(17), ionotropic subunits; P2Y(1,2,4,6,11,12), metabotropic receptors) has been cloned from vertebrate tissues. In this rapidly expanding field, one of the main current challenges is to relate the cloned P2 receptor subtypes to the diverse physiological responses mediated by the pharmacological phenotypes of native P2 receptors. Unfortunately, subtype-selective P2 ligands, especially potent and selective antagonists, have been only slowly forthcoming, and this acts as a considerable impediment to progress. However, a number of new P2 receptor antagonists have recently been described which to some degree are more potent and more selective than earlier antagonists like suramin or pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). This work moves us closer to the ideal goal of classifying the recombinant and native P2 receptor subtypes on the basis of antagonist profiles. This review begins with a brief account of the current status of P2 receptors and their ligands. It then focuses on structure-activity relationships of PPADS and suramin analogues and will finish with a brief discussion of some related therapeutic possibilities.