THE rod and cone transducins rue specific G proteins originally thought to be present only in photoreceptor cells of the vertebrate retina(1-4). Transducins convert light stimulation of photoreceptor opsins into activation of cyclic GMP phosphodiesterase (reviewed in refs. 5-7). A transducin-like G protein, gustducin, has been identified and cloned from rat taste cells(8). We report here that rod transducin is also present in vertebrate taste cells, where it specifically activates a phosphodiesterase isolated from taste tissue. Furthermore, the bitter compound denatonium in the presence of taste-ceh membranes activates transducin but not G(i). A peptide that competitively inhibits rhodopsin activation of transducing also blocks taste-cell membrane activation of transducin, arguing for the involvement of a seven-transmembrane-helix G-protein-coupled receptor. These results suggest that rod transducin tranduces bitter taste by coupling taste receptor(s) to taste-cell phosphodiesterase. Phosphodiesterase-mediated degradation of cyclic nucleotides may lead to taste-cell depolarization through the recently identified cyclic-nucleotide-suppressible conductance(10).