1. 1. Changes in ATP and phosphoprotein turnover observed when liver slices are suspended in media low in Na+ are in part due to influx of Ca2+. 2. 2. These changes are reversed by addition of Na+, and the amount of Na+ required for reversal is dependent on the concentration of external Ca2+. 3. 3. Lesser amounts of Na+ are required to stimulate ATP turnover than are required for phosphoprotein. 4. 4. Li ions fail to stimulate phosphoprotein turnover, though they may increase the activity of ATP. 5. 5. Na+, Ca2+ and the drugs ouabain and promethazine compete for phosphoprotein groups. 6. 6. K ions appear to be necessary for a dephosphorylating reaction of the phosphoproteins observed within 30 sec of K+ addition. This is blocked by ouabain and promethazine. 7. 7. The metabolism of ATP and phosphoproteins in liver cells is thus dependent on a balance between the activities of Na+, Ca2+ and K+.