In this series of irradiated animals, one lobe of each thyroid was put ]j ixative so that the quantitative histochemical (Feulgen staining) ve total amount of DNA on individual nuclei could be made,as done in earli «periments. Since cells normally divide when the DNA of the nucleus reache ve diploid value, the finding of considerably more than this amount of DNA istochemical method) in the large bizarre nuclear forms made the thymidine qbservations 1 the same animals of great value. A complete survey was made of a large mumber of nuclei in a microscopic 2asuring and classifying all nuclei by size and determining the distributi. abel. This showed that the abnormally large nuclear forms contained the -ten and were thus preparing for mitosis more frequently than other cells. The parallel histochemical data using the Feulgen stain and microspec 1 these same tissues showed that thiouracil caused a marked increase in DNA dclei as well as increase in the nuclear size. When this stimlus wes seviously irradiated glands, there was an increase in the variability of clear volume of these glands. The greater variability was attributable to resernce of the unusually large muclei which in the parallel observations > have a predisposition to take up the thymidine. immarizes this work: DNA and The following publicati osbyns, B.M., Rudd, Ann E., and Sanders, Mary A.: Desoxyribonucleic Acid (D 1 the Radiated and Stimulated Thyroid Gland, Endocrinology 81: 1-13, 1967. When the technique for clearly visualizing chromosames in human cells b ineulating leukocytes began to be developed in this country, Dr. Neil MacintEC >llaboration in the study of a patient with functioning metastatic yyroic whom we had treated with large doses of 1317 some years ,before and p seat again with very large doses. Although the oftT pa by us,che had had none in the preceding six years\| ly been iven large doses of t Py us, she had had none in the preceding six years. t very frequent intervals during,the first 24 hours and less frequently + days. The concentration of 131y in the blood, the leukocyte count and the differential sunt were determined at the same time. Spreads of chromosomes from miltiplel[cultures id frem many different individuals cells in those cultures were prepared ample of blood. An attached table (Figure 10) shows how the incidence of yomalies climbed to as high as 44% between 6 and 12 hours after the dose of iven. Based on a very extensive experience of observations on 2, 885 nor, 1e same technique on patients who had been exposed to other types of radiatibr assed, following the administration of isir, the radioactivity in the ci ation fell