wo ft = ronically, othe~s received it acutely before sacrifice, and still ot ceived none. Each animal was given tritiated thymidine four hours b crifice, so that contact radioautographs might be made to determine d how many cells possessed nuclei that were preparing for mitosis. me instances the rats were also given minute trace doses of 1131 to e function of the thyroid before they were sacrificed. The radioaut r tritiated thymidine were not prepared until this I131 had completely cayed. Some animals tere killed early in the course of the expe ther additional data on the supramaximal surge of DNA formation that]had en observed in previous experiments two months after the I131 was dy weights, thyroid weights and thyroid function, as measured by Irt3} ke, as well as gross changes in thyroids, are ali being determined af the me of sacrifice. These experiments nave been in progress for about one year. us far selected for sacrifice have been 34 and 9 months. Intefvals Animals will be erificed at more frequent intervals as the expected time for the oc neoplasms arrive. They will be sacrificed at intervals for 2 to 2] ° : ars. It is hoped that the intervals elected for sacrifice will give dioautographs at the time when the first signs of the development of ars gone by. As the neoplasms begin to develop, evidence for diffe rates of synthesis of DNA in different parts of the thyroid will be ught. At the time of the most recent sacrifice of animals, no gross idence of tumors had yet «appeared. clear Changes in Human Radieted Thyroid Tissue Over the years there have been opportunities to procure by surgic ans, samples of thyroid tissue from patients previously treated with yi3l, ving firmly established the method of Feulgen staining and tita crospectrophotometry on animal thyroids in this laborator;;(3), the thodology for procuring and processing the tissue was set into operatic ne human material was to become available. Thus, the quantitative m@asure— ne in the past on animals. Over a period of almost 10 years, thyrotc ssues from 13 1121 treated patients have been obtained for this s addition, & tissues from thyroids previously i & controls were obtained. subjectedto x-ray. rac Ten of the 13 1431 patients were subjected 3 operated on for persistence of hyperthyroidism; in two instances 3 obtained at prompt post mortem examination. Recently the final s tissue 3in method, it was found that somewhat fewer of these radiated tissue splayed bizarre nuclear forms than was_ observed in our previous radié nan thyroids described many years agn(2), However, four of 13 1231 | tients showed an abundance of the bizarre nuclear forms in extranod 3ssue. One of the most obvious had been given propylthiouracil before 2 operation because of continued hyperthyroidism. The use of this dz ich acted as a stimulus to the patient's thyroid, may have behaved shion comparable to our animal experiments where an abundance of bize slear forms developed when a similar stimulus was applied. In this dz