THE experimental literature, which we reviewed in a previous article (1), shows that normal thyroid tissue is resistant to X-rays. The clinical literature reveals practically nothing concerning the effect of X-rays on the hyperplastic gland but does indicate that X-ray treatment benefits, at least temporarily, some 60 per cent of patients afflicted with hyperthyroidism. No observations have been made to ascertain if X-rays may prevent hyperplasia. Bensley (2) has shown that, if opossums, when confined to cages, are fed a high protein meat diet, the thyroid undergoes marked hyperplasia which is quite evident in two weeks. He could not prevent this hyperplasia with iodine, but it did cause the hyperplastic gland to store colloid. Hence, we had at hand a method for determining if X-rays may prevent hyperplasia of the thyroid. Methods Dosage.—The same dosage used in our previous study (1), which was used by B. H. Orndoff, for the treatment of hyperthyroidism, was employed. For our equipment the factors were as follows: ma., 4; sphere gap, 130; peak, 177; filter, 0.25 mm. Cu plus 1 mm. Al; portal, 5 × 8 cm.; distance, 50 cm.; time, 55 min.; yielding 385 r units. One series consisted of one dose given at weekly intervals for four weeks (four doses). A month was allowed to elapse before a second series was given. Experiments.—Sixteen opossums were obtained from Texas. One was killed on the day of arrival and the thyroid which was removed, weighed, and examined histologically was found to be normal. Eight were treated with X-ray, seven being used as controls. Three of the eight (Lot 1) treated were started on meat and X-ray treatment at the same time. The other five (Lot 2) were started on meat immediately after the conclusion of the first series. Results Since Bensley found that the thyroids were normal on receipt of the opossums and remained so when the animals were kept on a proper diet, we sacrificed only one of our opossums to determine the state of the thyroid on coming into the laboratory. It was found to be normal and a section of it is shown in Figure 1. The average weight of the glands of nine opossums, found by Bensley to be histologically normal, was 157 milligrams. Lot 1.—In this lot, X-ray treatment was started at the same time as the meat diet. A study of Table I shows that the glands of the controls, the animals that received meat but no X-ray treatment, were markedly hyperplastic. Follicles were rare. The measurements show that the glands were grossly enlarged, while the glands of the animals that received X-ray treatment were hyperplastic, but contained numerous small follicles. The measurements of the glands show that they were smaller than the controls. Lot 2.—In this lot the meat diet was not started until after the first series of X-ray treatments. In general the results are the same as those obtained in Lot 1. The difference in the measurements of the treated and untreated glands corresponds with the weights (Table I).