The binding of ADP to chloroplast ATPase (CF1) has been measured by the chromatographic method of Hummel and Dreyer which allows low affinity binding determinations. Besides about 1.5 endogenous ADP molecules that are irreversibly bound to CF1or slowly exchangeable in the experimental conditions, ADP binds to CF1in the presence of Mg2+with an apparent unique dissociation constant of 64 μM, up to a total of 6 ± 0.5 moles/mole, when Mg2+is in excess over ADP. Under these conditions, the major part of the bound nucleotide is in the metal complexed form. Metal free ADP also binds to CF1, with a dissociation constant of 5 to 15 μM, measured by competitive inhibition of ATPase activity, at low Mg2+concentration.