In 19 children with chronic renal insufficiency sodium, potassium and magnesium in plasma and erythrocytes were measured before and after haemodialysis. Creatinine and the acid-base status were also determined. The mean predialysis values of sodium in plasma were within normal limits, in erythrocytes in the lower normal range. Potassium was elevated in plasma, and in erythrocytes within the normal linits. Magnesium was elevated in both plasma and erythrocytes. The changes during dialysis are described. Statistically significant relations of investigated variables are also described. The results are discussed.