Our earlier studies of the effects of roentgen radiation on growing bone (1) and the method of accomplishing accurate small-field collimation of the beam (2) have been previously reported. This paper expands the roentgen findings to include independent irradiation of the metaphyses and epiphyseal lines (physis) of the ulna and radius. The changes described in this report will be further investigated by histologic, chemical, and other methods. Mongrel dogs averaging ten weeks of age were irradiated with single doses of 800 or 1,500 r measured in air, at 220 kv, with a 7.7 mm. Al h.v.l. In all studies, the higher the dose of radiation, the earlier and more marked the changes, and more persistent the residua. All findings described in this study represent changes not seen in the opposite (control) leg. Metaphysis No significant difference was observed between the reaction of the ulnar and radial metaphyses (Fig. 1). The irradiated areas became radiodense in about two days after administration of 800 or 1,500 r. Density and definition increased for a few days, then faded in about two weeks. Areas given 800 r faded completely in three to four weeks after treatment, while those receiving 1,500 r continued to show some density as long as the stability of the implanted metal pins permitted them to be filmed—up to eight weeks. Irradiation of the ulnar and radial metaphysis did not influence growth. The epiphysis, epiphyseal line, and adjacent metaphysis were unaffected in their rate of growth or roentgenographic appearance. Epiphyseal Line Irradiation of the epiphyseal line necessarily includes a margin of the metaphysis proximally and epiphysis distally. In the radius, the initial change was radiodensity of that portion of the metaphysis included in the field. The density occurred twenty-four and forty-eight hours after treatment with 800 rand 1,500 r, respectively; the same intervals as when the metaphysis alone was treated. Maximum radiodensity was reached in three to seven days. In approximately ten days, a limited zone of growth cessation was noted in the irradiated area. This fact can be appreciated in Figure 2, which shows the untreated metaphyseal portions along the epiphyseal line growing away from the irradiation defect. In addition, the irradiated radius demonstrates an overall decrease in longitudinal growth rate of approximately 10 per cent; this change began about five days after treatment. Radiation of the ulnar epiphyseal line produced different findings. The initial changes were a loss in density of the metaphyseal tip included in the irradiated field and an increased width of the epiphyseal line. These findings were first definite at two days and progressed further from the third through the fifth day. The rounding of the metaphyseal tip and widening of the epiphyseal line are illustrated in Figure 3.