Since the discovery of Na/K-ATPase, evidence has accumulated to suggest that 1 mol of ATP hydrolysis occurs via the Na+-occluded ADP-sensitive phosphoenzyme, the K+-sensitive phosphoenzyme and the K+-occluded enzyme accompanying active transport of 3Na+ and 2K+ according the Post-Albers scheme. However, some controversial issues have arisen concerning whether the functional unit of the enzyme is an αβ-protomer or a much higher oligomer, which would be related to the mechanism of transport, either sequential or simultaneous. Detailed studies of oligomer interaction and the reactivity of the enzyme and a comparison of the extent of phosphorylation with ligand-binding capacities in the presence or absence of ATP hydrolysis and others strongly suggest that the functional unit of the enzyme in the membrane is a tetraprotomer, (αβ)4. They also suggest that each reaction intermediate of the Post-Albers scheme, respectively, reflects half of the site property of the intermediate and that another half binds ATP. These data may be useful not only to answer the long-standing question of whether the mechanism functions in the presence of both Na+ and K+ but also contribute to a better understanding of the mechanism of P-type pump ATPase in general.