given for a few days to enhance the uptake of the carrier-free dose of 131;, ich is counting in representative animals. [thyroid injected intraperitoneally.The series of animals have ranged from 35 to 250 amimals. Several dose levels have been used in each series. The animals are individualiy marked. In vivo counting of the uptake by each thyroid is measured at 24 hours and sdime animals are sacrificed at this time so that the in vivo measurements may be checked Hy in vitro The thyroids are weighed and an average weight determined. The dose in rads may thus be calculated for each animal. Animals were grouped according to the doses of +311 administered. Each group was sulfiivided into those which received chronic thiouracil in the drinking water, those which, are given thiouracil only for 5 days before sacrifice and those given nothing but. 7” Controls received no treatment at all. Aside from those animals initially sacrificed to check precise uptake off thyroid weight, the animals were sacrificed at infrequent in grvals for the first year and from time to time up to 25 years. A small trace dose of I in addition tc the tpitiated thymidine was given to the animals four hours before sacrifice. Autoradiograph promptly prepared from microscopic sections of the thyroid serve to identify ny local areas in the gland where 18!y utilization is different the rest of the gfard. Because the half life of 1911 is mich shorter than the 9H in tritiated thymidine, the 131] disappears from the microscopic sections more rapidly and is essentially] gone after two months. New autoradiographs then prepared from other microscgpic sectionf, adjacent to those used to demonstrate 1511, show only the location of the H-thymidine] over the muclei that were preparing to divide. Subsequent comparisons of the two autpradiogrephs reveal two types of information about the same cluster of cells which appear both preparations. In order to conserve animals in later experiments, some were anesthetize@ and the thyroid explored about the time neoplasms were first expected to appear. If farly nodularity was found, the animals were sacrificed several days later, after the usual presacrific= preparation. This was done not only to conserve valuable animalg, but also to give a better idea as to when neoplasms begin and an appreciation of Bow fast they grow. The first full series of animals (85 males) were divided into groups amnd[civen 9, 5, 10, or 50 uc of 1317, delivering from 30,000 to 300,000 rads to the thyfoid. In this series the 1311 was not given until the animis had grown to 110 to 135 grams. Jnfortunately, there was a very low incidence of neoplasms (6 identified) in }hi series, although the long term survival was not particularly good. Animals wii ceceived the larger doses of 1311 had almost complete destruction of the thyrgid. In animals that received lower doses, some thyroid tissue could be seen on gross|dissection. There was only minimal architectural distortion of the follicles in this rangd. A secomd series of rats similar to the first was prepared. Since the yidld of teoplasms had been poor in the juvenile rats approaching maturity in the firs] series, wanlings were used. By the time 1317 was given they weighed 55 to 85 grams. They received 40,000 to 400,000 rads. At about 1 year an epidemic of pneumonitis qwept through the animal farm destroying animals more by groups than by rarxiom. (It was no@ necessarily the most heavily radiated animals that died.) Some tumors developed in survilprs, suggesting that radiation applied to younger animals would yield a higher prolability of eoplasms. tke