Changes in magnesium metabolism, along with those in sodium, were investigated in 17 patients with Graves' disease (14 females and 3 males, mean +/- SD, 44.8 +/- 12.2 years) and their relationship to plasma levels of thyroid hormones were assessed before and after treatment. Each patient was studied in hyperthyroid state and euthyroid state after treatment. Each patient was studied in hyperthyroid state and euthyroid state after treatment with methimazole. Treatment with methimazole increased the magnesium concentration both in erythrocytes (2.00 +/- 0.18 vs. 2.08 +/- 0.24 mmol/l cells, P < 0.05) and in serum (0.72 +/- 0.12 vs. 0.84 +/- 0.11 mmol/l, P < 0.001) but both urinary output and fractional excretion of magnesium decreased significantly (P < 0.05 and P < 0.001, respectively). The erythrocyte sodium concentration decreased with treatment (10.7 +/- 2.6 vs. 8.1 +/- 1.1 mmol/l cells, P < 0.001) but the serum sodium remained unchanged (140.9 +/- 1.9 vs. 140.9 +/- 2.1 mmol/l, NS). Urinary excretion of sodium also decreased with treatment (P < 0.05), but only changes in indices of magnesium metabolism (decrease in renal fractional excretion, rise in serum level) correlated significantly with those of the thyroid functions with treatment. These observations clearly indicate that in Graves' disease, the magnitude of magnesium metabolism alteration is closely related to the extent of the increase in thyroid hormones in plasma.