-4A small trace dose of 1311 in addition to the tritiated thymidine fhas been given to the animals four hours before sacrifice. Autoraciographs promptly prepared from microscopic sections after the - ‘oval of the thyroid serbe to identify any local areas in the gland where 1317 utilization ts differant fron the rest of the gland. Because the half life of 131t is much shorter than the fi in tritiated thymidine, the 1311 disappears from the micrescopic sedtions more rapidly and is sone after tvo or three months. lew cutcradiozraphp then prepared from other microsecpic sections, adjacent to thcse used to dembnstrete 317 show only the location of the 311-thymidine. Subsequent compariso two autoradiographs from microscopic sections, which are almost identi reveal two types Of information about the same cluster of cells which both preperations. We have several larve grours of rats whose thyroids have been i with various doses cf 1312 which cause relatively little or no obvious of thyroid tissue, these doses ranging from 0 to 40 microcuries of +31I varying degrees of uptake depending on the experimental design. Each individually identified and its uptake determined for future refererce. these animals were given an antithyroid drug chronically following tne produce hyperplasia. Others are stimulated acutely only for a few days sacrifice to see how much mitotic activity could be produced. Still ot remained unstitulated, except for the intrinsic stizwlus which the ani received as a result of diminution in hormone output by its own radiat The rats of this strein (Charles River strain, since 1947) very rarely spontanecusly. These animals have been observed up to two years. of the abkpear in Bestruction with ahi wes some: of PS+I to before Although most animals in the current series have been sacrificerd be described. Even though neoplasms are induced by goitrezgens alone, produced soone> and mere readily if relatively small amounts of 131I given. hafe first been tt has been suspected that the age of the animal at the time 1311 Is given is ixportant in the frequency end cpeed with which neoplasms are produced.] In previous experiments with juvenile rats approaching sexual maturation, the ccecurfence of necplases following 131] treatment and chronic administration of antith}roid substances was fairly infreguent. In fact, the yield of rats with necp with which to work was discouragingly low. A large series of weanling rets $5 to 85 gms has ylelced a somewhat better occurence of neoplasms. These, Like previous animals, were given Remingtcn diet to enhance the uptake and make hyperplasic thyroids. In these. animals, the uptake of the dose was 25 to 40%. We are now well aleng in still another series in which very young | used by acquiring creenant females and injecting the litters with 1S1I were reasonably sure of the healthy survivors (15 days; 39 to 40 gn; nojRemington diet) In this series, the uptake was about 10%. Animals were sacrificéd in hours (to determine averace thyroid weights and to measure actual uptak@ to check invivo counting), days, «ewexs and months up to almost two years. The several hundred animals in these series ranged from 500 rads to 40,000 fac a few considerably higher. These ranges have been explored in order tojfind the level at which neoplasms wore most readily produced. Obvicusly doses caused too much destructicn left cells incapable of replacing themselves. 5S other lighter doses were too snall to cause any discernible untoward effects.] In thse experiments the higher doses of about 20,000 rads o> more have produced|consicerable