1. In eight patients being treated by chronic dialysis the relation of magnesium metabolism to the magnesium concentration of the dialysate was investigated. During four periods of 2–6 weeks the magnesium concentrations of the dialysis fluid were 0.44±0.15 mg/100 ml, 0.70±0.24 mg/100 ml, 1.83±0.18 mg/100 ml and 1.97±0.21 mg/100 ml.
2. During all periods the concentration of plasma magnesium and ultrafiltrable magnesium was above normal. The concentration of erythrocyte magnesium was below normal with the lowest concentration in the bath, while all other dialysate concentrations yielded readings above normal values.
3. During dialysis the concentration of plasma magnesium and ultrafiltrable magnesium dropped only during the periods with low concentrations of magnesium in the dialysate. During dialysis with the lowest concentration the values for erythrocyte magnesium showed a significant rise whereas with high concentrations the concentration fell significantly. The changes in erythrocyte magnesium proved to be independent of the calcium content of the dialysis fluid.
4. The maximum loss of magnesium to the bath solution amounted to 710 mg, with the magnesium content of the bath being only 0.27 mg/100 ml. The net transfer of magnesium to the blood of the patient amounted to 102 mg with a dialysate magnesium concentration of 1.86 mg/100 ml.
5. It is concluded that low dialysate magnesium concentrations result in erythrocyte magnesium deficiency whereas high concentrations cause magnesium overloading. A magnesium concentration of 0.7 mg/100 ml is suggested for the bath solution used in regular dialysis treatment.