TRPM5, a member of the superfamily of transient receptor potential ion channels, is essential for the detection of bitter, sweet, and amino acid tastes. In heterologous cell types it forms a nonselective cation channel that is activated by intracellular Ca 2؉ . TRPM5 is likely to be part of the taste transduction cascade, and regulators of TRPM5 are likely to affect taste sensation. In this report we show that TRPM5, but not the related channel TRPM4b, is potently blocked by extracellular acidification. External acidification has two effects, a fast reversible block of the current (IC 50 pH ؍ 6.2) and a slower irreversible enhancement of current inactivation. Mutation of a single Glu residue in the S3-S4 linker and a His residue in the pore region each reduced sensitivity of TRPM5 currents to fast acid block (IC 50 pH ؍ 5.8 for both), and the double mutant was nearly insensitive to acidic pH (IC 50 pH ؍ 5.0). Prolonged exposure to acidic pH enhanced inactivation of TRPM5 currents, and mutant channels that were less sensitive to acid block were also less sensitive to acid-enhanced inactivation, suggesting an intimate association between the two processes. These processes are, however, distinct because the pore mutant H896N, which has normal sensitivity to acid block, shows significant recovery from acid-enhanced inactivation. These data show that extracellular acidification acts through specific residues on TRPM5 to block conduction through two distinct but related mechanisms and suggest a possible interaction between extracellular pH and activation and adaptation of bitter, sweet, and amino acid taste transduction.All animals have evolved mechanisms by which they sense chemicals in the environment and discriminate the nutritious from the noxious. In mammals, taste cells signal the presence of at least five distinct types of chemicals that give us the perceptions of bitter, sour, sweet, salty, and amino acid tastes. Two of these modalities, salty and sour, detect ions (Na ϩ and H ϩ , respectively), and both modalities may be mediated directly by ion channels on the apical surface of taste cells (1). The other three modalities are mediated by distinct G proteincoupled receptors (2-4) that signal through a common transduction cascade, for which both phospholipase C 2 and the ion channel TRPM5 are essential elements (5). It is likely that upon binding of taste molecules, sweet, bitter, and amino acid taste receptors initiate a signaling cascade that activates phospholipase C 2, leading to the breakdown of phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate and diacylglycerol and the release of Ca 2ϩ from intracellular stores (6, 7). TRPM5 may be the final element of this cascade, converting the biochemical signal transduction pathway into an electrical signal sent to the brain (5, 8). The essential role of TRPM5 in multiple taste pathways suggests that chemicals that modify its activity could have profound effects on taste sensation.TRPM5 belongs to the transient receptor potential (TRP) 1 fa...