K + interactions with a rat brain ( Na + + K +)-dependent ATPase and the associated K +-dependent nitrophenyl phosphatase activity were examined. Classes of sites for K + were distinguished, initially, on the basis of affinity estimated by kinetic analysis in terms of K 0.5 (the concentration for half-maximal activation), and by K +-accelerated enzyme inactivation by F −, which permits evaluation of a dissociation constant for K +, K D. Moderate-affinity sites (“α sites”), with a K D near 1 mM, were demonstrable for the phosphatase activity and for the “free” enzyme. High-affinity sites (“β sites”), with a K D near 0.1 mM, were seen for the overall ATPase activity and under conditions in which enzyme phosphorylation by substrate also occurs. Further differentiation between α and β sites was made in terms of (i) the characteristic changes in affinity with pH, and (ii) the efficacy of Li + relative to K +, Rb +, Cs +, and Tl + at these two classes of sites. Low-affinity sites (“gg sites”) through which K + inhibits enzymatic activity were also detectable, with a K D around 140 mM. These data are incorporated into a model for the reaction sequence to accommodate both transport processes and certain K +/ATP antagonisms.