The effect of different cellular free-energy states on the uptake of methyl alpha-D-glucopyranoside, an analogue of glucose, by the Escherichia coli phosphoenolpyruvate:carbohydrate phosphotransferase system was investigated. The intracellular [ATP]/[ADP] ratio was varied by changing the expression of the atp operon, which codes for the H+-ATPase, or by adding an uncoupler of oxidative phosphorylation or an inhibitor of respiration. Corresponding initial phosphotransferase uptake rates were determined using an improved uptake assay that works with growing cells in steady state. The results show that the initial uptake rate was decreased under conditions of lowered intracellular [ATP]/[ADP] ratios, irrespective of which method was used to change the cellular energy state. When either the expression of the atp operon was changed or 2,4-dinitrophenol was added to wild-type cells, the relationship between initial phosphotransferase uptake rate and the logarithm of the [ATP]/[ADP] ratio was approximately linear. These results suggest that the cellular free-energy state, as reflected in the intracellular [ATPI]/[ADP] ratio, plays an important role in regulating the activity of the phosphotransferase system.