This chapter discusses the kinetic models of Na-dependent phosphorylation of Na, K–ATPase from rat brain. Na+-dependent kinetics of the enzyme have indicated that multiple Na+ ions interact with it. Kinetic models for Na+ interaction on two or three equivalent sites and two or three nonequivalent sites have been proposed in the chapter. The chapter presents a further analysis of these models. The approach presented may provide a useful tool to kinetically characterize Na+ interactions with the enzyme in relation to active site E–P intermediates. The E–P versus Na+ data were analyzed by Scatchard plot, Hill plot, double reciprocal plot, and a multiple equilibrium titration curve based on the equation discussed in the chapter. A Scatchard plot of the data was markedly curved inward toward the origin, indicating multiple Na+ interactions. Double reciprocal and Hill plots were nonlinear, which were divided into three regions by inspection and straight line fitted by linear regression in each region. The two or three nonequivalent-site models gave a better fit than the equivalentsite models.