1. Serum, urinary and erythrocyte magnesium concentrations were measured in groups of premenopausal, postmenopausal and oophorectomized women. 2. Serum and urinary magnesium were both significantly higher in postmenopausal and oophorectomized women than in the premenopausal group. 3. Oestrogen therapy reduced both serum and urinary magnesium values in oophorectomized women to premenopausal concentrations. 4. Erythrocyte magnesium concentrations were not affected by menstrual status or oestrogen therapy.