Abstract 1. 1.|A search has been made for mechanisms which may exert a “fine” control of the glucose-6-phosphate dehydrogenase reaction in rat liver, the rate limiting step of the oxidative pentose phosphate cycle. 2. 2.|The glucose-6-phosphate dehydrogenase reaction is expected to go virtually to completion because the primary product (6-phosphogluconate lactone) is rapidly hydrolyzed and the equilibrium of the combined dehydrogenase and lactonase reactions is in favor of virtually complete formation of the phosphogluconate. In fact, however, the reaction does not go to completion because glucose-6-phosphate dehydrogenase is inhibited by NADPH. 3. 3.|Measurements of the degree of inhibition (which is competitive with NADP + show that at physiological concentrations of free NADP + and free NADPH the enzyme is almost completely inhibited. This indicates that the regulation of the enzyme activity is a matter of the de-inhibition. 4. 4.|Among over 100 cell constituents tested only GSSG and AMP counteracted the inhibition by NADPH. The effects of AMP at physiological concentrations were very slight while GSSG was highly effective at concentrations which may be taken to occur physiologically. 5. 5.|The effect of GSSG was not due to the GSSG reductase activity of liver extracts because under the test conditions the activity of this enzyme was very weak, and complete inhibition of the reductase by Zn 2+ did not abolish the GSSG effect. 6. 6.|Pre-incubation of the enzyme preparation with GSSG in the presence of Mg 2+ and NADP + before the addition of glucose-6-phosphate and NADPH much increased the GSSG effect. 7. 7.|Dialysis of liver extracts and purification of glucose-6-phosphate dehydrogennse abolished the GSSG effect, indicating the participation of a cofactor in the action of GSSG. 8. 8.|The cofactor removed by dialysis or purification is very unstable. The cofactor could also be separated from glucose-6-phosphate dehydrogenase by centrifugation of liver homogenates in Amicon “centrifio” membrane cones. Some properties of the cofactor are described. 9. 9.|The hypothesis that GSSG exerts a fine control of the pentose phosphate cycle by removing the NADPH-inhibition of glucose-6-phosphate dehydrogenase is discussed. Although conclusive proofin support of the hypothesis is lacking (because it has not been shown that the concentrations of GSSG vary as postulated) it seems highly probable that the postulated control mechanism operates. It is pointed out that there are atuations where an enzyme must be inac ve although the substrate is abundant and the equilibrium favors the reaction. In such a situation a mechanism of the type postulated for glucose,6phosphate dehydrogenase-a special type of inhibition by one of its products (not caused by an equilibrium of the catalyzed process), and the relief of this inhibition by another metabolite, seems to be the only way of coping with the physiological requirements. A similar case is the inhibition of hexokinase by glucose-6-phosphate.