Gustatory information processing begins with taste bud cells,astr which are activated by sapid molecules via specific taste receptors and transmit their signals to gustatory afferent fibers. Taste bud cells are morphologically classified into 4 groups (Type I -IV cells), two of which are involved in gustatory sig-naling. Type II cells express sweet, bitter, and umami taste receptors and transduction components and respond best to sweet, bitter, or umami stimuli, suggesting that sweet, bitter, and umami tastes are detected by different sets of Type II cells. Type m cells express putative sour taste receptors and respond to sour or multiple taste stimuli, indicating that sour tastes are mediated by Typem cells. These data suggest that each taste quality could be discriminated among taste bud cells. Type II cells do not possess a conventional synaptic structure but they release ATP in response to taste stimuli. Typem cells have a synaptic structure and they release serotonin and norepinephrine but not ATP. Therefore, each taste cell may use distinct mechanisms and transmitters for signal transmission to gustatory nerve fibers.