ateral observations were made on these first two series or animais in studying the development of neoplasms. [Early sacrifices confirmed our ition that there is a temporary loss of mitotic activity shortly after as we previously reported. The abnormally large pupber of mitoses which ritrogen was given during the recovery phase from 4312 occurred again. lasms do not become manifested until much later, it may be that they is time. od series of animals, the age at which 151z was given was even lower being > 65 grams. Although a period of iodine deficient diet was agair used, the to 60% of dose, and the graded doses of 1I were reduced so that 5000 to were delivered to the thyroid, using 5 dose levels. A total of 19 animals plasms. Although the number of animals that developed tumors seems small, pt in mind that many were sacrificed before one year and tumors were seldom 14 months. Only 25 animals were allowed to survive after 14 months; 12 of ely proved to have neoplasms. rge series of animals was prepared. Since the yield of neoplasms had giving the 131t at a younger age in the preceeding series, this present arted by procuring pregnant females that had all been bred on the same ere 120 offspring. Ninety of these were given 131z on the 16th day of life oneal injection. Thirty served as various kinds of controls. Males and separated after weaning. Icdine deficient diet could not be used here. d dose delivered to these animals ranged from 300 to 17,000 rads, based on yroid weight of 7 mg. Since the neoplasms tended to appear more often in 2 range in the preceeding group of arimals, lower doses were used here. these thyroids were extremely small, so that considering the range of the itively more of the beta radiation caused ionization outside the gland. A liscrete neoplasms were found in this series. One of the non-radiated n thiouracil chronically developed a neoplasm. There were 8 neoplasms in received only radiation and 20 had been given radiation and chronic Only 49 anima’ 3 were permitted to live longer than 12 months; 22 of these | 3 when sacrificed. Most of the neoplasms that developed without the stimuluq L occurred in the 7,000 to 17,500 rad dose range. Liateral observations have been made on these series of animals. Since midine radioautographs have been made on all thyroids in these 4 series, reat deal of information that is just now being appreciated retrospectively. 2 observations are as follows: Upon reviewing the non-radiated control 2d at each period of sacrifice it was found that when the young animals are y course of thiouracil, the degree of rise in mitotic index (number of S$ per high power field) is quite remarkable. The young unstimilated control 2d 0.3 to 1.6 labeled nuclei while those given thiouracil showed 14.0 to h power field. In contrast the same observations on resting animals 1% to ge may reveal only one or two labeled nuclei in several cross sections of a d lobe. When the latter are stimulated under the same conditions the maximal clei labeled may be less than one per high power field. es of 2300 to 2800 rads, slight temporary impairment to mitotic activity ultimately a maximal weight of thyroid could be produced with chronic thiou