TRPM8 is a nonselective cation channel activated by cold and the cooling compounds menthol and icilin (Peier et al., 2002] from pH 7.3 to approximately pH 6 abolished responses to icilin and cold stimulation but did not affect responses to menthol. Icilin concentrationresponse curves were significantly shifted to the right when pH was lowered from 7.3 to 6.9, whereas those with menthol were unaltered in solutions of pH 6.1. When cells were exposed to solutions in the range of pH 8.1-6.5, the temperature threshold for activation was elevated at higher pH and depressed at lower pH. Superfusing cells with a low subactivating concentration of icilin or menthol elevated the threshold for cold activation at pH 7.4, but cooling failed to evoke [Ca 2ϩ ] i responses at pH 6 in the presence of either agonist. In voltage-clamp experiments in which the intracellular pH was buffered to different levels, acidification reduced the current amplitude of icilin responses and shifted the threshold for cold activation to lower values with half-maximal inhibition at pH 7.2 and pH 7.6.The results demonstrate that the activation of TRPM8 by icilin and cold, but not menthol, is modulated by intracellular pH in the physiological range. Furthermore, our data suggest that activation by icilin and cold involve a different mechanism to activation by menthol.