1. A search was 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. The glucose 6-phosphate dehydrogenase reaction is expected to go virtually to completion because the primary product (6-phosphogluconate lactone) is rapidly hydrolysed and the equilibrium of the joint dehydrogenase and lactonase reactions is in favour of virtually complete formation of phosphogluconate. However, the reaction does not go to completion, because glucose 6-phosphate dehydrogenase is inhibited by NADPH (Neglein & Haas, 1935). 3. Measurements of the 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 de-inhibition. 4. Among over 100 cell constituents tested only GSSG and AMP counteracted the inhibition by NADPH; only GSSG was highly effective at concentrations that may be taken to occur physiologically. 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. Preincubation 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. Dialysis of liver extracts and purification of glucose 6-phosphate dehydrogenase abolished the GSSG effect, indicating the participation of a cofactor in the action of GSSG. 8. The cofactor removed by dialysis or purification is very unstable. The cofactor could be separated from glucose 6-phosphate dehydrogenase by ultrafiltration of liver homogenates. Some properties of the cofactor are described. 9. The hypothesis that GSSG exerts a fine control of the pentose phosphate cycle by counteracting the NADPH inhibition of glucose 6-phosphate dehydrogenase is discussed.