Many collateral observations were made on these first two series df animals in the course of studying the development of neoplasms. Early sacrifices Fonfirmed our former observation that there is a temporary loss of mitotic activity shortly after I is given as we previously reported. The abnormally large pupber af mitoses which appear when goitrogen was given during the recovery phase from I ocd ed again. Although neoplasms do not become manifested until much later, it may bel that they started at this time. In a third series of animals, the age at which 1317 was given was bven lower being 28 days; 40 to 65 grams. Althcugh a period of iodine icient diet wap again used, tr uptake was 25 to 60% of dose, and the graded doses of I were reduced so that 5000 tc 200,000 rads were delivered to the thyroid, using 5 dose levels. A tothl of 19 animals developed neoplasms. Although the number of animals that developed tumpbrs seems small, it mist be kept in mind that many were sacrificed before one year and thmors were seldc found before 14 months. Only 25 animals were allowed to survive after 24 months; 12 of these ultimately proved to have neoplasms. A 4th large series of animals was prepared. Since the yield of nebplasms had increased by giving the 1311 at a younger age in the preceeding series,] this present rinineeneetadnnber de “Seertnh str.seulewnedinniairnians eeetel bps teas: day. There were 120 offspring. by intraperitoneal injection. Ninety of these were given 31I on the] 16th day of lif Thirty serve. as various kinds of controis. Males und Females were separated after weaning. Iodine deficient diet could not fe used here. The actual rad dose delivered to these animals ranged from 300 to 17,00P rads, based on an average thyroid weight of 7 mg. Since the neoplasms tended to appeap more often in the lower dose range in the preceeding group of animals, lower doses we used here. Furthermore, these thyroids were extremely small, so that considering ep range of the beta ray relatively more of the beta radiation caused ionization outsidp the gland. A total of 29 discrete neoplasms were found in this series. One of the nh radiated controls given thiouracil chronically developed a neoplasm. There were] 8neopiasms in animals that received only radiation and 20 had been given radiation an thiouracil . Only 49 aninsls were permitted to live longer than 12 months;22 of these had neoplasms when sacrificed. Most of the neoplasms that developed wifhout the stim of thiouracil cccurred in the 7,000 to 17,500 rad dose range. Many collateral observations have been made on these series of animals. Since tritiated thymidine radioautographs have been made on all thyroids in tlese 4 series, there is a great deal of information that is just now being appreciated retrospectively Some of these observations are as follows: Upon reviewing the non-radid*ed control animals killed at each period of sacrifice it was found that when the qung animals are given a 5 day course of thiouracil, the degree of rise in mitotic index|(mmber of . labeled cells per high power field) is quite remarkable. The young unsfimulated contro. animals showed 0.5 to 1.6 labeled nuclei while those given thiouracil sHowed 14.0 to 35.0 per high power field. In contrast the same observations on resting animals 1% to 2 years of age may reveal only one or two labeled muiclei in several crogs sections of a whole thyroid lobe. When the latter are stimulated under the same cordjtions the maxim number of nuclei labeled may be less than one per high power field. For doses of 2300 to 2800 rads, slight temporary impairment to mitdtic activity occurred but ultimately a maximal wight of thyroid could be produced chronic thic