A group of patients whose dietary potassium was unrestricted and who received 12–18 h of Kiil dialysis twice weekly against a bath containing no potassium, had body potassium concentrations (total body potassium/intracellular volume) of 7.6% lower than normal. Despite marked hypokalemia at the end of dialysis, surprisingly few electrocardiographic changes were seen. Another group of subjects, dialyzed for 5–6 h thrice weekly against a bath containing 1 mEq/liter of potassium in a Dow dialyzer, showed more marked electrocardiographic abnormalities despite smaller alterations in transmembrane potassium gradients. Rapidity of establishment of potassium gradients is important as well as magnitude. The following changes occur in a single dialysis: 100 mEq of cell potassium and 20 mEq of extracellular potassium leave the body; 100 mEq of extracellular sodium enter the cells and 415 mEq of extracellular sodium leave the body; 3.5 liters of water leave the extracellular fluid, 2.5 liters into the bath and 1 liter into the ce