Taste receptor cells detect chemicals in the oral cavity and transmit this information to taste nerves, but the neurotransmitter(s) have not been identified. We report that adenosine 5'-triphosphate (ATP) is the key neurotransmitter in this system. Genetic elimination of ionotropic purinergic receptors (P2X2 and P2X3) eliminates taste responses in the taste nerves, although the nerves remain responsive to touch, temperature, and menthol. Similarly, P2X-knockout mice show greatly reduced behavioral responses to sweeteners, glutamate, and bitter substances. Finally, stimulation of taste buds in vitro evokes release of ATP. Thus, ATP fulfils the criteria for a neurotransmitter linking taste buds to the nervous system.