After frog taste cells were adapted to 1 mM quinine-HCl (Q-HCl) for 10 sec, modification of receptor potentials in the taste cells induced by salt, acid, sugar and bitter stimuli was studied with microelectrodes. The phasic component of receptor potentials induced by 0.1 M NaCl, KCl, NH4Cl and MgCl2 was enhanced following adaptation to Q-HCl. The rate of rise of receptor potentials in response to the salts was increased after Q-HCl adaptation. The amplitude and the rate of rise of receptor potentials induced by 1 mM acetic acid were larger after Q-HCl adaptation than after water adaptation. The amplitude of phasic component and rate of rise of receptor potentials for 0.5 M sucrose after Q-HCl were the same as those after water. The amplitudes of tonic receptor potentials for 1 mM Q-H2SO4, brucine and picric acid after Q-HCl adaptation were the same as those after 1 mM NaCl adaptation. Correlation coefficient between taste cell responses induced by 1 mM Q-HCl and 1 mM Q-H2SO4 was very high, but those between 1 mM Q-HCl and 1 mM brucine responses and between 1 mM Q-HCl and 1 mM picric acid responses were low. This indicates that Q-HCl and Q-H2SO4 bind to the same receptor site, but brucine and picric acid bind to different receptor sites to which Q-HCl does not bind.