In recent years much interesting information about the mechanism of the Na⁺-K⁺ activated ATPase has been obtained from investigation of the K⁺-activated phosphatase activity which appears to be catalysed by the same enzyme. Also several studies have indicated that a K⁺-activated p-nitrophenylphosphatase activity is intimately related to the ATPase activity. And then the exact relation of p-nitrophenylphosphatase activity to Na⁺-K⁺ ATPase activity is not known. The effects of some ions and drugs on the p-nitrophenylphosphatase activity of the rat brain were investigated and the results were summarized as follows. 1. The p-nitrophenylphosphatase was stimulated markedly by low concentrations of K⁺, while the activity was activated slightly in the presence of Na⁺ and oligomycin. 2. Addition of both ATP and Na⁺ caused a remarkable increase in the activity of the K⁺-dependent phosphatase at low concentrations of K⁺. 3. In the presence of Na⁺ and low concentrations of K⁺, oligomycin activated the p-nitrophenylphosphatase. 4. O1igomycin inhibited the stimulation of the enzyme activity caused by Na⁺+ATP. 5. Ouabain inhibited the K⁺-dependent p-nitrophenylphosphatase activity more in the presence of ATP and Na⁺ than in their absence. 6. Quinidine inhibited both Na⁺-K⁺ ATPase and p-nitrophenylphosphatase. These inhibitory effects of the drug were partially antagonized by increasing K⁺ concentrations. The sensitivity of the K⁺-dependent p-nitrophenylphosphatase to quinidine was greater than the that of Na⁺-K⁺ ATPase.