Intra- and extracellular levels of magnesium and potassium were determined in 16 subjects with insulin-dependent type I diabetes mellitus (IDDM) and 30 healthy controls. Subjects with IDDM had lower levels of magnesium in muscle biopsies (p less than 0.001), plasma (p less than 0.001), and mononuclear cells (p less than 0.05), and higher urinary excretions of magnesium (p less than 0.01), and lower levels of potassium in muscle biopsies (p less than 0.001), and erythrocytes (p less than 0.05), as compared with those in controls. Magnesium hydroxide (500 mg/day) was administered orally to the diabetics. The levels of magnesium and potassium in muscle biopsies increased (p less than 0.001; p less than 0.001), while the plasma levels of magnesium and the urinary excretions of magnesium increased only temporarily, during 21 weeks of treatment. The requirements of insulin were reduced (p less than 0.001) during the course of the study, whereas the levels of glycosylated hemoglobin (HbA1c) and glucose were not changed. The findings indicate that administration of magnesium hydroxide is useful to treat muscular magnesium and potassium deficiency in diabetics.