1. Ox heart phosphofructokinase catalyses isotope-exchange reactions at pH6.7 between ADP and ATP, and between fructose 6-phosphate and fructose 1,6-diphosphate, the latter reaction being absolutely dependent on the presence of the magnesium complex of ADP. 2. The reaction kinetics are hyperbolic with respect to substrate concentration for both exchange reactions (within the experimental error). 3. The influence of pH, AMP and citrate suggests that the fructose 6-phosphate-fructose 1,6-diphosphate exchange is subject to effector control, and is abolished by dissociation of the enzyme. 4. These results are discussed in relation to the reaction mechanism of the enzyme.