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The increase in serum calcitonin during pregnoncy raises the problem of the role of oestrogens in the secretion of calcitonin. The authors show that oestrogen causes, in castrated rats, an increase in calcitonin activity as measured in the plasma by a biological method. This effect of injected oestrogen does not occur if the castrated animal is also submitted to thyroidectomy. The effects of progesterone studied on a small number of cases, seem to be less marked. This report shows that oestrogens have a synergistic effect with calcitonin in the maintenance of calcium within the bone. This is of importance during pregnancy and after the menopause.