Acid-sensing is associated with both nociception(1) and taste transduction(2). Stimulation of sensory neurons by acid is of particular interest, because acidosis accompanies many painful inflammatory and ischaemic conditions. The pain caused by acids is thought to be mediated by H+-gated cation channels present in sensory neurons(3-5). We have now cloned a H+-gated channel (ASIC, for acid-sensing ionic channel) that belongs to the amiloride-sensitive Na+ channel(6-11)/degenerin(12-14) family of ion channels. Heterologous expression of ASIC induces an amiloride-sensitive cation (Na+ > Ca2+ > K+) channel which is transiently activated by rapid extracellular acidification. The biophysical and pharmacological properties of the ASIC channel closely match the H+-gated cation channel described in sensory neurons(3,15,16). ASIC is expressed in dorsal root ganglia and is also distributed widely throughout the brain. ASIC appears to be the simplest of Ligand-gated channels.