For the first time it is shown that 2 anterior pituitary hormones—prolacting and the growth principle—do not cause the thyroid hypertrophy which characteristically follows the injection of various pituitary extracts. Hyperplasia of the normally developed thyroid following pituitary administration is a specific response to the gonad-stimulating hormone, or to another pituitary derivative having very similar solubilities. Good gonad-stimulating preparations do not invariably induce an increase in thyroid weight in doves and pigeons.