THE key carbohydrate intermediate, glucose 6-phosphate, which may follow several alternative metabolic pathways including the 6-phosphogluconate oxidative cycle and the glycolytic sequence, enters the latter route by the phosphoglucose isomerase reaction. This reaction, in which glucose 6-phosphate is reversibly converted into fructose 6-phosphate, could conceivably play a part in the regulation of carbohydrate metabolism, although it has seldom been considered from this point of view, doubtless because of the extremely high activity of the enzyme in most tissues and because the reaction has so far appeared to be very little affected by extraneous influences. The present evidence suggesting that the enzyme is strongly inhibited by 6-phosphogluconic acid, an intermediate in the direct oxidative pathway, may therefore be of interest in connexion with studies on the mechanism responsible for the partitioning of carbohydrate between the direct oxidative pathway and the glycolytic route.