-/l6iTads in children and 350 in the adults. Oniy 2 cr 19 children expes ed under the 2 sé 7 = Tm 17 of 10 deveioped frank clinic al Wepetnyroidism and that required a nu mner of vears tc become evident. This raised a question whether the estimate of the dose in the Mar- Shallese was too low. Both children were irradiated at about 1 yeaf of age. of these has developed nodules. Neither Presumably, cellular damage was su fficiently great to preclude thyroid cell replication. Of the other 17 children who were exposed, 15 did not display clinical hypothyroidism but have developed neopl hsms and been operated. Our experience finding carcinomas in radioiodine treated pati ts in the National Cooperative Thyrotoxicosis Therapy Follow-Up Study seems to be qui e contrary to the Marshallese experience. An unusually small number of carcinomas considerably fewer than would have been expected, considering the number of occult malignant lesions found in Graves' disease treat s found; in fact, elatively larger by surgery. Such incidental lesions found at surgery might have been expected to g w if unknowingly left in place and 131, treatment had been given. tween the doses The difference received by the Marshallese thyroids and those of Graves' disease fi s considerable. Graves' disease patients usually receive 5,000 to 14,000 rads whi the adult Mar- shallese received an estimate of 550. Furthermore, the hyperplasti gland of Graves' disease would be expected to be far more sensitive to the radiati m than the normal glands of the Marshallese. Our animal experiments and the most recent studies of other investigators using rats seem to have suggested strongly that it is the smaller doses circumstances under which neoplasms arise. Indeed the effects ar 0 f£ 131, that produc sufficiently mild so that no microscopic architectural distortion is recognized in he thyroids most 1 Ciined to develop neoplasms. In our studies we have shown that there was a failure of the moderately irradia’ Tat thyroid to hypertrophy when stimulated by giving an antithyrs id substance. We hh: also shown in such thyroids that there is a marked reduction in fhe capacity of such