It has been shown by Lacassagne1 and more recently by Burrows2 that carcinoma of the breast may be produced in male mice by the administration of large doses of estrogenic hormone. These observations are of great importance because this is the first instance of direct association of malignant neoplasia with an excess of a hormone normally produced by the organism. The question immediately arising is: Does spontaneous carcinoma of the breast in mice result from hypersecretion of estrogenic hormone over a long period?
As a result of the classic works of Allen,3 and of Allen and Doisy and their associates,4 the secretion of estrogenic hormone may be studied readily in the intact mouse by observation of the estrus cycle. Proestrus and estrus correspond to the period of ripening and the period of maximum activity of the graafian follicle. Metestrus and diestrus are periods in which no