The protective effect of ATP, ADP and GTP against the inactivation of Ca2+ + Mg2+ -dependent ATPase by the thiol reagent NBD-chloride is used to calculate the apparent dissociation constants (K'D) of nucleotide enzyme complexes on the basis of a simple kinetic model. The K'D-values of the complexes with Mg-ATP (80 micrometer) and Mg-GTP (500 micrometer) are found to be rather close to their Km-values in the high concentration range supporting maximum activity. The requirement of the occupancy of the low affinity site by Mg ATP for a high rate of the Ca2+ transport system is explained in terms of the flip-flop mechanism established earlier for the analogous Na+ + K+-transporting ATPase system.