The changes in the concentration of respiratory intermediates in potato tuber slices during the transition from quiescence to physiological activity, characterized by a strong rise in respiratory metabolism, have been investigated, in order to study the mechanism of respiratory activation. The initial concentrations of glucose-1-phosphate, glucose-6-phosphate and fructose-6-phosphate are, respectively, 4X10-6 M, 8X1O-5 M and 1,5X10-5 M; these do not show any marked change during the respiratory activation. The same is true for inorganic phosphate. Pyruvate concentration decreases slowly from the initial value, 5X10-5 M, to 2,5X10-5 M at the end of the incubation of the slices (8 hours). Sucrose level initially decreases in connection with the increase of fructose and glucose; then the concentration of these metabolites reaches a constant value. These results show a remarkable constancy of the concentration of all glycolytic intermediates measured. The occurrence and the nature of mechanisms controlling the flow rate of glycolysis is discussed. It seems possible, on the basis of the observations reported, that the changes in the concentration ratios of ATP, ADP and AMP might operate as a control mechanism at different sites of the glycolytic pathway.