Mg deficiency was experimentally induced in 3 calves given Mg-deficient semi-synthetic milk (10 mg of Mg/l of milk). Three control calves received the same milk supplemented with Mg (190 mg of Mg/l of milk). Calcium and vitamin D contents of this milk were similar to that of natural milk. Daily weight gain in control calves was similar to that observed in calves given natural milk. The daily weight gain of Mg-deficient calves was not statistically different from that of control calves during the first 6 weeks of Mg-deficiency. Then clinical symptoms of Mg-deficiency appeared (loss of appetite, nervousness, tetany crises). In these Mg-deficienct animals a fall in plasma magnesium level, followed by hypophosphataemia and then by hypocalcaemia was observed. During the whole experimental period plasma CT and plasma PTH levels were not statistically different in Mg-deficient and in control calves. These results show that the hypocalcaemia observed in Mg-deficient calves was due neither to an increase in CT secretion nor to a decrease in PTH secretion. Nevertheless, the parathyroid glands from these animals seem unable to react satisfactorily to the hypocalcaemia induced by Mg-deficiency.