Taste buds contain two types of cells that directly participate in taste transduction--receptor (Type II) cells and presynaptic (Type III) cells. Receptor cells respond to sweet, bitter and umami taste stimulation but until recently the identity of cells that respond directly to sour (acid) tastants has only been inferred from recordings in situ, from behavioural studies, and from immunostaining for putative sour transduction molecules. Using calcium imaging on single isolated taste cells and with biosensor cells to identify neurotransmitter release, we show that presynaptic (Type III) cells specifically respond to acid taste stimulation and release serotonin. By recording responses in cells isolated from taste buds and in taste cells in lingual slices to acetic acid titrated to different acid levels (pH), we also show that the active stimulus for acid taste is the membrane-permeant, uncharged acetic acid moiety (CH₃COOH), not free protons (H⁺). That observation is consistent with the proximate stimulus for acid taste being intracellular acidification, not extracellular protons <ital>per se</ital>. These findings may also have implications for other sensory receptors that respond to acids, such as nociceptors.