The concentration of magnesium in muscle was determined and a standardized magnesium load test was performed in 21 patients, who 4 to 10 years previously had undergone intestinal bypass operations for severe obesity. The plasma concentration and 24-h urinary excretion of magnesium were also studied. Basic urinary excretion of magnesium and muscle magnesium were significantly lower in patients compared to healthy controls, while no differences were found in plasma magnesium. A slight negative correlation between muscle magnesium and retained magnesium was demonstrated (r = -0.51, P less than 0.05). Patients with magnesium retention greater than 20 per cent showed a significant decrease of magnesium retention after treatment with magnesium chloride mixture. Four patients with primarily low muscle magnesium all demonstrated an increment in the amount of magnesium in muscle after treatment. The load test described can be applied as a screening test in diagnosing magnesium deficiency.