Modification of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity from Lactococcus lactis was undertaken during batch fermentation on lactose, by adding various concentrations of iodoacetate (IAA), a compound which specifically inhibits GAPDH at low concentrations, to the culture medium. As IAA concentration is increased, GAPDH activity diminishes, provoking a decrease of both the glycolytic flux and the specific growth rate. This control exerted at the level of GAPDH was due partially to IAA covalent fixation but also to the modified NADH/NAD+ ratio. The mechanism of inhibition by NADH/NAD+ was studied in detail with the purified enzyme and various kinetic parameters were determined. Moreover, when GAPDH activity became limiting, the triose phosphate pool increased resulting in the inhibition of pyruvate formate lyase activity, while the lactate dehydrogenase is activated by the high NADH/NAD+ ratio. Thus, modifying the GAPDH activity provokes a shift from mixed-acid to homolactic metabolism, confirming the important role of this enzyme in controlling both the flux through glycolysis and the orientation of pyruvate catabolism.