1. Plasma and erythrocyte magnesium concentrations have been determined in fifty-five patients on regular haemodialysis and in thirty-two healthy subjects. The dialysis fluid magnesium concentrations of the former were 0·58, 0·9 and 1·73 mm.
2. There was a linear correlation between the plasma and erythrocyte magnesium concentrations of normal subjects and dialysed patients whose dialysis fluid contained 0·58 and 0·9 mm-magnesium and who were treated by dialysis for at least 3 months. This correlation was otherwise independent of the time the patients had been treated by dialysis.
3. There was a linear correlation between the dialysis fluid and the predialysis plasma magnesium concentrations in all dialysed patients, except that, when the dialysis fluid magnesium was increased, a certain period of treatment was necessary before the new equilibrium was reached.
4. An unusually high magnesium concentration in the dialysis fluid for regular haemodialysis may benefit such patients in view of the known effect of an increased plasma magnesium concentration upon the parathyroid function.