Hypokalemia occurs in up to 60% of patients on peritoneal dialysis (PD) for end-stage renal disease. It can be associated with poor clinical outcomes, including malnutrition and death. New evidence points to a link between hypokalemia and the risk of peritonitis. The serum Na+ concentration is determined by the ratio of exchangeable Na+ and K+ to total body water. Hyponatremia may be caused by excessive thirst, a particular problem in patients on PD. Consistent with the relationship among the serum sodium concentration, total body cation content, and total body water, hyponatremia may also be caused by total body potassium depletion. Ultra-low sodium dialysis fluid may prove to have an important role in management of fluid overload and hypertension in patients on PD. Surprisingly, it is not associated with significant hyponatremia, for reasons that remain to be elucidated.