Effects of some bitter-tasting compounds on frog taste receptors were examined by recording glossopharyngeal nerve responses. The order of effectiveness of the compounds was quinine greater than brucine greater than formanilide greater than caffeine greater than urea. When the effects of quinine, brucine and caffeine on electrical responses in taste cells were examined, they all produced a depolarization associated with an increased input resistance. The action of the three compounds on taste receptors therefore, operates with a similar mechanism. The electrical responses in cells, produced by quinine, progressed slowly with time. Such effects with quinine are similar to those with procaine. After adaptation to quinine, the nerve responses to various chemical stimuli were gradually reduced in magnitude, while the electrical responses in taste cells during stimulation by chemicals became smaller. The mechanism of the effects of bitter stimuli are discussed in light of recent findings on the interaction of bitter stimuli with lipid monolayers and the extraction of lipid from bovine taste papillae by bitter stimuli.