1. Roentgen dosages of from 800 to 1,200 r were directed over selected joints of sixweek-old dogs. The treated area included the joint, epiphysis, and adjacent metaphysis.

2. The epiphyseal cartilage was profoundly affected, and retardation of growth occurred.

3. The articular cartilage of the treated joints showed frank microscopic evidence of degenerative changes, and was not so thick as in the controls. These changes were not sufficient to interfere with joint function during the period of the experiment, and were not apparent on gross examination.

4. There was no evidence of significant abnormality in the adjacent cortical or cancellous bone, synovial membrane, joint capsule, skin, or pariarticular soft tissues.

5. Mild bowing of the treated fore limb occurred in eight of twelve animals. No such deformity occurred in the hind limbs.

6. Although shortening of the treated bone occurred in each instance, there was evidence of stimulation of growth at the untreated epiphysis of the treated long bone. The cause of this stimulating effect is not apparent.

7. The clinical use of roentgen irradiation of growing epiphyses to control longitudinal growth inequalities in children must he considered as experimental, and fraught with some potential dangers. Whether the danger is greater than is incurred in surgical procedures, such as lengthening or shortening of the lower extremity and epiphyseodesis, must await further investigations.