The effect of sodium and potassium ions on intracellular acid production and acid excretion by glycolyzing cells of Streptococcus mutans was examined. S. mutans NCTC 10449 grown under glucose-limited and strictly anaerobic conditions in a continuous culture system was loaded with bis(carboxyethyl)-carboxyfluorescein, a pH-sensitive fluorescent dye, washed and suspended in 0.00-0.30 M NaCl/KCl solution. The dye allowed for the continuous monitoring of intracellular pH while proton excretion was measured simultaneously with a pH-stat. Sodium ions inhibited and potassium ions, at low pH, accelerated the amount of measurable acid excreted extracellularly. In the presence of both NaCl and KCl, proton excretion following the addition of glucose was slightly higher or similar to that observed in the presence of 0.15 M KCl alone. Sodium and potassium ions did not affect the proton-ATPase enzyme or the intracellular level of ATP, suggesting that these ions did not directly effect proton pumping activity itself. The inhibition of proton excretion by sodium ions was considered to have probably occurred as a result of an indirect inhibition of proton-ATPase activity by the low intracellular pH induced by sodium ions.