The heart muscle is always exposed to radiation when cases of mediastinal tumor, carcinoma of the left breast, or bronchial and lung tumor are treated in this region. In view of the importance of this organ, it is surprising how little we know regarding changes that might take place in the heart tissue following irradiation. Through the experiments of Roffo,1 Krontowski2 and Schubert,3 we learned that the embryonic heart is rather resistant to roentgen rays. Roffo could not find any injury caused by irradiation with roentgen rays of long or short wavelength, in cultures of chicken embryo hearts. Krontowski demonstrated that there is a marked difference between the susceptibility of the embryonic organism as a whole and the individual embryonic organ: where a certain dose is fatal to the organism, no permanent injury results to the single organ or cell. Tissue cultures of the heart grew equally