The pH effects on frog gustatory responses to alkali-metal and alkali-earth-metal chloride salts were examined using single fungiform papilla preparations. Responses to 0.1-0.5 M NaCl were clearly dependent upon the pH of the stimulating solutions. The responses increased as the pH decreased from 6.5 to 4.5 and were almost completely suppressed at pH above 6.5. There was no significant difference in the pH dependency of the response among alkali-metal chlorides. HCl solutions elicited only a poor response when the water response was suppressed by the simultaneous presence of a low NaCl concentration. Responses to alkali-earth-metal chlorides varied in their pH dependency. Response to CaCl2 was slightly affected by pH changes from 4.5-9.0, response to SrCl2 was considerably suppressed in the alkaline region and responses to BaCl2 and MgCl2 were strongly suppressed at pH above 6.5. BeCl2 solutions showed less marked stimulating effects over the pH range tested. The differences in pH dependency imply 2 kinds of receptor site: pH-insensitive sites responsible for the Ca response and pH-sensitive sites responsible for the Na response. A cross-adaptation test appeared to support this possibility. Assuming that the pH effect mentioned is related to changes in the state of ionization of the receptor molecule, the pKa of the ionizable group responsible for the Na response was about 5.5.