1. 1. A cation-stimulated ATPase has been labelled with [32P]ATP. 2. 2. It is shown that 95% of the label passes into a phosphoprotein which has been identified by the isolation of radioactive phosphorylserine. 3. 3. Phosphorylation of the protein is stimulated by Na+ and requires the presence of Mg2+. 4. 4. K+ stimulates dephosphorylation of the phosphorylated protein, Mg2+ is required for this step also. 5. 5. Strophanthin G inhibits both the Na+- and K+-stimulated steps. The latter is more sensitive to the glycoside. 6. 6. Observed rates of phosphorylation and of dephosphorylation of the radio-active phosphoprotein are compatible with observed rates of the cation-stimulated ATPase of the same preparation.