zhe long term series of stu¢ies on the morpnclogical and physiolcf&issi effects of +712 on se thyroid have contisued along severai lines of investiga = as de- scribed in the mcre recent progress reports. The observations continue =. both man and animals. Previous reporzs have provided detailed descriptiong =:= the work. In mst of these areas we continus to assemble data as the opportunitiB:=and time Provide. The clinical data inclsc= studies on the Marshallese whose orisery effects were caused by radioiodines from <ie thermonuclear bomb fallout. The areas of study which we reve discussed in considerable detai¥j reports and continue to be investizated are as follows: ” <= previous Variations in clinical responsiveness to 131; therapy for Graves' di . The detailed collection c= radiation effects continue on high], selected cooperative patients. The appearance of significant amcots of mono (M)) and dtiodotynos39 C7) anda rise in iodide in the circulatioz._=ollowing therapeutic doses of | Appearance of M, and D» <s followed by thyroid failure within = very few months. The failure of lightly radiated tcyroid cells to hypertrophy when stigk:_ated by giving goitrogenic substances. In animais increase in cel height may occur through the acti rt. of TSH release but the gland does not increase in size (weight J because <ne capacity for cell miltiplication is inhibited. The question is whe r cmose cells that can and do multiply =re the source of neoplasms and whetfh= reduction in the capacity for replication of normal cells reduces the The actual buildup in the amount sf DNA in the nucleus of radiated during the interphase but the faiiure of cells to divide. The buildup we have show. may exceed two times the diploid <cid cells We know from previous animal experiments and suspect fram observati subjects that there is a differerre between the effect of a relativ on clinical ~arge dose of <srinsic nuclear 2h damages the 131] which causes both impairment to the synthesis of thyroxine and damage (that precludes subsequern= cell division) and the small dose nucleus but does not seriously impair thyroxine production. The problem is to determine these respective doses. <e. The result ~ animal and man 7 is bizarre nuclear forms seen in many radiated thyroids in <= the thyroid There is a dose level of +941 in man that results in a latent fail long after all of the isotope is zone (years). This seems to be attribu-able to intrinsic nuclear damage whjc-. does not de- stroy the cell nor its hormone production but does result in[<= intrinsic damage which 1s ultimately lethal when mitosis is attempted. Following a modest dose of 1317 <3 rats and a latent period during be shown to be synthesizing DNA =rom thymidine, a surge of many cel has been found. Since the relative number of cells incorporating thymidine 4 ==4 no cells can synthesizing DNA s the controls, the surge of attempted proliferecive activity may have a special meafi=s. The question is whether a relative physiologi-sl failure of thyroxine output actifie <hrough a TSH mechanism is responsible for such. a surge.