1. 1. Rat brain lipoprotein can bind Na+. 2. 2. ATP increases the binding of Na+ but the effect is due to removal of Mg2+ from binding sites. 3. 3. EDTA is even more effective than ATP in increasing Na+ binding by the protein. 4. 4. Na+ binding by protein is not necessarily related to the cation-stimulated ATPase activity of the protein. 5. 5. Other ions such as K+, Li+, Mg2+ bind as effectively to the protein as Na2+. 6. 6. The present methods are inadequate to determine if Na+ is bound to phosphorylated intermediate in cation-activated ATPase systems.