Effects of Na+, K+, and nucleotides on Mg2+-dependent phosphorylation of (Na+ + K+)-dependent adenosine triphosphatase by Pi were studied under equilibrium conditions. Na+ was a linear competitive inhibitor with respect to Mg2+ and a mixed inhibitor with respect to Pi. K+ was a partial inhibitor; it interacted with positive cooperativity and induced negative cooperativities in the interactions of Mg2+ and Pi with the enzyme. Adenyl-5'-yl (beta, gamma-methylene)diphosphonate, a nonhydrolyzable analog of ATP, interacted with negative cooperativity to inhibit phosphorylation in competition with Pi. ATP was also a competitive inhibitor. Na+ and K+ acted antagonistically, Na+ and nucleotides inhibited synergistically, and K+ and nucleotides were mutually exclusive. In the presence of ouabain, when nucleotides were excluded from the site inhibiting phosphorylation, a low affinity regulatory site for nucleotides became apparent, the occupation of which reduced the rate of dephosphorylation and the initial rate of phosphorylation of the enzyme without affecting the equilibrium constant of the reaction of Pi with the ouabain-complexed enzyme. The regulatory site was also detected in the absence of ouabain. The data suggest that catalytic and transport functions of the oligomeric enzyme may be regulated by homotropic and heterotropic site-site interactions, ligand-induced slow isomerizations, and distinct catalytic and regulatory sites for ATP.