ATP receptor-mediated responses in peripheral and central neurons have many characteristics which suggest that ATP may act as a fast neurotransmitter. While the receptors underlying these responses have properties which are similar to other ligand-gated ion channels which mediate fast neurotransmission, the nature of their calcium permeability and the rapid breakdown of ATP to adenosine may confer unique properties on ATP mediated synaptic transmission. The evidence that ATP acts as a fast neurotransmitter is reviewed and the properties of ATP and its receptor channels are discussed in terms of synaptic transmission.