The changes of magnesium utilization during pregnancy in rat and the influence of hydrocortisone acetate on it was studied. In the late days of gestation the Mg balance and magnesium content of bone increased significantly. This may be due to a greater requirement at this stage and to bone acting as a store for this cation. Hydrocortisone acetate at a dose of 4 mg/100 g body weight/day decreased the magnesium balance but did not modify the bone content of this mineral. The hormone had a catabolic effect on Longissimus dorsi muscle, producing a loss in magnesium. The effect is directly proportional to the duration of treatment.