VARIOUSe environment which result in therapeutic benefit in patients with metastatic mammary cancer are illustrated. The use of calcium excretion as a practical means of evaluating tumor growth in bone is shown. The results of surgical castration, bilateral adrenalectomy, and hypophysectomy are presented. A physiological basis for these methods of treatment is presented by demonstrating that certain hormones of the ovary, adrenal, and pituitary glands are important in maintaining the growth of mammary cancer. The therapeutic effects of cortisone, estrogens, and androgens are illustrated, with emphasis on the situations in which these hormones are most likely to be of benefit. The sequence of therapeutic measures which is most likely to produce optimum benefit is outlined.