When the rate of glucose addition to nongrowing Streptococcus bovis cell suspensions was increased, the fermentation was homolactic, fructose-1,6- diphosphate (FDP) increased, intracellular inorganic phosphate (P(i)) declined, and the energy-spilling rate increased. ATP and ADP were not significantly affected by glucose consumption rate, but the decrease in P(i) was sufficient to cause an increase in the free energy of ATP hydrolysis (ΔG'p). The increase in ΔG'p was correlated with an increase in proton motive force (Δp). S. bovis continuous cultures (dilution rate of 0.65 h- 1) that were provided with ammonia as the sole nitrogen source also had high rates of lactate production and energy spilling. When Trypticase was added as a source of amino acids, lactate production decreased; a greater fraction of the glucose was converted to acetate, formate, and ethanol; and the energy- spilling rate decreased. Trypticase also caused a decrease in FDP, an increase in P(i), and a decrease in Δp. The change in Δp could be explained by P(i)-dependent changes in the ΔG'p. When P(i) declined, ΔG'p and Δp increased. The ratio of ΔG'p to Δp (millivolt per millivolt) was always high (>4) at low rates of energy spilling but declined when the energy- spilling rate increased. Based on these results, it appears that Δp and the energy-spilling rate are responsive to fluctuations in the intracellular P(i) concentration.