The effects of thyra-parathyroidectomy with subsequent thyroxin replacement have been studied in two young and two mature sheep. Observations made included the effects of the operation on the absorption, excretion and plasma levels of calcium, phosphorus and magnesium, when the animals were kept on diets of different calcium and phosphorus levels. It was found that in the young sheep postoperative hypocalcaemic tetany occurred unless the dietary intake of calcium was extremely high. In the mature sheep there was a temporary drop in plasma calcium levels after the operation, but tetany did not occur irrespective of the calcium intake. It is maintained that in the young sheep the hormones of the thyroparathyroid complex governing calcium and phosphorus and magnesium metabolism, play a much more essential role than they do in mature sheep. Actions of these hormones which have been demonstrated are: (1) increase of calcium absorption in the young animal and inhibition of absorption in the mature ones; (2) increase of urinary phosphate excretion and retention of calcium.