The uptake of methyl alpha-D-glucopyranoside by the phosphoenolpyruvate-dependent phosphotransferase system of Salmonella typhimurium could be inhibited by prior incubation of the cells with glycerol. Inhibition was only observed for glycerol preincubation times longer than 45 s and required the preinduction of both the glucose and the glycerol-catabolizing systems. Larger extents of inhibition by glycerol correlated with higher intracellular levels of glycerol kinase when the glp regulon had been induced to different extents. Preincubation with lactate did not inhibit methyl alpha-D-glucopyranoside uptake significantly, although both lactate and glycerol were oxidized by the cells. The cellular free-energy state of the cells (intracellular [ATP]/[ADP] ratio) was virtually identical for lactate and glycerol preincubation, suggesting that the inhibition of phosphotransferase-mediated uptake was not a metabolic effect. In vitro, phosphotransferase activity was inhibited to a maximal extent of 32% upon titrating cell-free extracts with high concentrations of commercial glycerol kinase. The results show that uptake systems that have hitherto been regarded merely as targets of the phosphotransferase system component IIA(Glc) also have the capacity themselves to retroinhibit the phosphotransferase system flux, presumably by sequestration of the available IIA(Glc), provided that these systems are induced to appropriate levels.