-21- All animals were individually and peimanently identified. Rather than measure the accumulation and disappearance of thyroidal 131 I on 30 or 40 representative animals and from this calculate an effective half-life (which in the past served as an estimate for all if the sample was relatively uniform), the actual Hata have been acquired individually on all animals. Although the a weights at the time of irradiation mst be estimated on the basis of 30 or 40 animal these weights were quite wmiform. initially sacrifice With individual effective half -lives known, the actual rad dose for each animal is much more precisely known. | In this current series several milliliters of blood were drawn by direct heart puncture for TSH assays sever: times during the 18 month period to see when the TSH rose foll it did occur. T, was administered to sane of these animals. There were several new and further objectives in this expe 1. irradiation, if ent: To sample the TSH levels in animals several times during an]/18 month interval between irradiation and sacrifice. If thyroid fimction fails, 1 probably will occur after a long interval, considering the rad dose used (approximat ly the same dose rar. in the thyroid was achieved in this experiment as in the past). If it does fail, hor does the date of failure relate to the development of a neoplasm’ 2. > To see if there is an inhibitory effect of thyroxine (T,) a ministration on the development of neoplasms. T, was given to half of the anima’ Is in this group. TS 4 assays served to test the campleteness of the TSH suppression by Ty The dose of Ty was adjusted accordingly. 3. To appraise changes in the pituitary by weighing the gland and assaying its TS content and relating the results to the presence of thyroid tumo; ’s, rad dose receive by the thyroid and the effects of Ty suppression. 4. Since neoplasms of the breast occasionally occur spontaneou: ty Tats and since their occurrence had seemed high in some irradiat in Sprague-Dawl: Tats in some past experiments when females were used, another objective was to rec@rd the occurrence o: breast tumors and to remove each breastlesion to preserve the life of the animal. ~~