The therapeutic action of stilbestrol in the human being is under investigation by the Council on Pharmacy and Chemistry of the American Medical Association. This case is reported as a contribution toward defining its effect on the human species. One of the established biologic effects of the naturally occurring estrogens is their growth-stimulating action on certain tissues and organs, one of which is breast tissue. It has also been experimentally established that when estrogenic substance is administered in sufficient amounts it has an inhibitory effect on anterior pituitary lobe trophic functions, among which are the production of growth hormone and the gonadotropic factor. A limited amount of clinical evidence supports these experimental observations. I1have induced gynecomastia in males by administering the natural estrogenic substance. The tissue was studied and reported on by Dr. Charles F. Geschickter.2The production of breast development in the female by the administration