ATP is a co-transmitter in the central and peripheral nervous system. Extracellular ATP exerts its effects via ionotropic (P2X), as well as metabotropic receptors (P2Y). P2X receptors are involved in fast excitatory synaptic signalling by ATP, whereas the role of P2Y receptors in synaptic transmission is unclear. Seven different mammalian P2X receptor subunits (P2X1-7) have been cloned to date. This article gives an overview about the distribution of these P2X receptor subunits in the nervous system. A comparison is made between the pharmacological properties of recombinant receptors and natively occurring neuronal P2X receptors by means of electrophysiological methods. The subcellular distribution of, developmental influences on, and interspecies differences between P2X receptors are also considered. It is concluded that the properties of native P2X receptors are best explained by a heteromeric assembly of different P2X receptor subunits.