The existence of two kinds of high energy phosphorylated intermediate, EP, with and without bound ADP, in the reaction of Na+-K+-dependent ATPase [EC 3.6.1.3] was indicated by the following results. 1. Only one type of EP, i.e., that for which decay is unaffected by the addition of ADP, was observed in the presence of high concentrations of Mg2+. On adding K+, a fraction of the EP decayed rapidly, and then the time course of EP decomposition followed first-order kinetics. The initial rapid decay of EP on addition of K+ was accompanied by the formation of an equal amount of ATP. The amount of ATP formed was equal to that calculated on the basis of the mechanism previously proposed by Kanazawa et al., that the equilibrium between an enzyme-ATP complex and EP shifts toward formation of the enzyme-ATP complex on adding K++ 2. When NEM-treated enzyme was used in the presence of low concentrations of Mg2+, two types of EP were observed. One was sensitive, and the other was insensitive to ADP. The decay of the former type was accelerated by the addition of ADP. The percentage of ADP-sensitive EP was zero in the initial phase of the reaction after adding ATP, and increased with time to a steady-state level. 3. These two experimental results strongly support the modified mechanism for the Na+-K+-dependent ATPase reaction proposed by Kanazawa et al., in which the two types of EP in the ATPase reaction with and without bound ADP are both high energy forms. The results are in disagreement with the mechanism proposed by Fahn, Albers, Post, and co-workers in which high energy and low energy EP intermediates are formed sequentially in the reaction of transport ATPase.