Dialysis fluid containing a low concentration of magnesium ( ≤ 0.05 mmol/l) was used to treat 17 patients on maintenance haemodialysis for varying periods of time up to 5 years. Mean plasma and ultrafiltrable magnesium concentrations measured in these patients before haemodialysis did not differ significantly from control values; after dialysis, plasma and ultrafiltrable magnesium concentrations were reduced only temporarily. Erythrocyte magnesium concentrations were elevated both before and after haemodialysis: dialysis did not significantly alter the mean concentrations. There was no significant correlation between the length of time on maintenance haemodialysis and either plasma or erythrocyte magnesium levels. As the mean magnesium concentration of bone biopsy specimens was also significantly greater than the mean from the control subjects, there was no evidence that magnesium depletion had been produced in these dialysis patients.