The increasing occurrence of labeled nuclei in these young growingfrats did not progress beyond 10 hours and thereafter declined progressively at 14, 14, 20 and <& hours reaching a level of only 1/3 of the maximal level. This decline dftexr 10 hars was observed in both series of animals. The explanation for this is no Clear. it might be assumed that once the DNA of a nucleus is clearly labeled withB icient isotope and the two preducts of mitosis are readily identifiable, label@i cells wulc not disappear froma gland. Destruction of that cell by radiation from not seem plausible in this interval of time. tritium c of tritiate These experiments have furnished considerable information on the thymidine in studying cell division in the thyroid. They have yielded ¢ rowgh bid . They he approximation of the time required for mitosis to take place in the t clarified some uncertainties and broadened our perspective of experime Gesigns for studying the capacity for mitosis in the thyroid. Together with e Lie exrerime replicatt they have furnished some appreciation of the enormous capacity far cell in the young vs the old, but they have not yet furnished us with much i ormaticn on the life of the thyroid cell. The experiments have introduced more ques ioms than tr of the have solved but much of what has been learned car. be applied to the irradiated thyroid cell. OBSERVATIONS ON THE DEVELOPMENT OF THYROID NODULES IN POPULATIONS TO RADIOIODINE FALLOUT AS THEY RELATE TO STUDIES UNDER THIS Co LAD ah The Marshallese Study In February and March of 1969, the responsible investigator spent f ve weeks in e Mershalles the Marshall Islands participating in the annual review of thyroids of exposed to the fallout, particularly the radioiodines, from the thermon flea devise detonated on Bikini in March, 1954. The nodularity in the thyroids vas irst observe nine years after exposure among these people. By 1967 fourteen of sixty eigmit cf the =p requir most heavily exposed had developed nodules which were of sufficient con exploration. One carcinoma had been found. As a result of the survey e rly in 1969, five more individuals in this population were found to have developed t roic masses Four were from the more heavily expose d group cn which we felt should be explored. Rongelap atoll, i.e. several hundred to fourteen hundred rads estimated dbse to tte thyroid; and one on Utirik atoll who had probably received 30 rads. The qe were orcugi ause of cur back to the U.S. for special study at Brookhaven National Laboratory. interest in and laboratory studies of radiated thyroids, the patients ere brought <o institution in September, 1969 for surgical exploration. Three of the f @ proved to uK) . One of have lesions of the thyroid which were malignant (two Rongelap; one these had extensive regional metastases. A fourth patient possessed seve ral modules, one of which was very small and has prompted serious consideration that t his mignt also be malignant. Autoradiographs were immediately prepared. There wer > also a vert Seme ot of observations commonly made on tissues from our animals (not 3H thymidi ne). these thyroids had many mimite solid cellular hyperplastic lesions, none f wihich took up Significant amounts of radioiodine. Many of the lesions were papillar im struct iés (except and suggested papillary lesions of milticentric origin. All of these orms repezte the women from Utirik) showed many examples of the large bizarre nu More will I observed under this contract in e:nimals and humans that had received said later of stvdy of these tissues umder the subject of identification f sobtile changes in thyroid tissue produced by 431.