1. 1. Liver slices suspended in Ringer containing choline instead of sodium rapidly lose potassium to the medium. This loss is reversible upon removing the slices to a normal Ringer. 2. 2. Rapid changes in phosphoprotein and ATP turnover occur upon transfer of slices to choline - Ringer. The former substances show these changes more quickly than does ATP. 3. 3. Addition of small amounts of Na+ reverse the effects of the Na+-free medium on phosphate turnover. The changes brought about by Na+ precede the ion changes seen during recovery from incubation in Na+-free media. 4. 4. The phosphoprotein fraction has been identified by isolation and chromatography of phosphorylserine derived from the radioactive phosphoproteins. 5. 5. Drugs, known to inhibit ion movements in liver slices, dissociate the effect of Na+ on phosphoproteins from that on ATP. It is suggested that Na+ transport is mediated by at least two separate steps. 6. 6. The phosphoprotein fraction affected by brief incubation of slices in Na+-free media is shown to be non-mitochondrial.