labeled miclei was only 0.4% but when such animals were injected with TSH, the incidence rose to 74. Thus, with this technique it was possible to demonstrate that cpll division in the untreated young rats was occurring at a rate almost as fast as in theladult animals whose thyroids had been stimlated with TSH. Histologically, in theladult animals, the thyroids were truly in a resting state. After considerable experimentation it was found that the most desirable interval of time between the injection $f the tritiated thymidine and the sacrifice of the animals was 4 hours. There was[no uptake of thymidine in cells which were resting. Even in this 4 hour interval therg were same instances of labeled pairs of nuclei lying in juxtaposition with approximately equal amounts of DNA in each. This gave a general idea of the rate at which cell division took place once the preparation for mitoses had begun. A manuscript describjng these observations has been prepared but not published. Dobyns , B.M., and Sanders, M.A.: A Comparison of the Effect of TSH on the totic Activity in the Thyroids of Young Growing Rats and Adult Rats Using Tritiated midittle. (Manuscript not published; later manuscripts incorporated the technique and later findidgs went far beyond these). Rat thyroids made hyperplastic by an iodine deficient diet display many nuclei containing the tritiated thymidine. When in a state of iodine deficiency dodes of 0, 5, or 50 pic of I were given to groups of rats te ce various degreds of radiation damage. Twenty hours after giving 5 pic of I (av. uptake 37%; 240 day half-life; 2640 Rads), the number of labeled nuclei were noticeably reduced.| At this early hour the thyroids of animals given 50 pe showed an almost complete absqnce of labeling of nuclei, indicati that the process of mitosis had been inte days after these doses of 1311, the administration of thiouracil caused mark ed. Twesrty-tc increase in labeling in nonirradiated controls and somewhat less labeling in the 5 uc kanimals, but the 50 pic animals had not recovered the ability for labeling. Recovery fran impairment to mitosis as studied by the labeled thymidine has been of special] interest. After more lengthy periods of recovery following radiation, some of the F ima from each group were given thiouracil for 9 to 11 days and tritiated nidine 4 hours before sacrifice. The results after various amounts of radiation and varioug periods of recovery are shown in Figure 9. At 2 months following the 5 pic dose of + HI there was an excessive number of labeled nuclei when thiouracil was given. The mupber of labeled nuclei was considerably greater than that which was produced in a nomirradiated animal. This seemed to be related to the previous observation that gross hypertrophy of the gland could still be produced by thiouracil administration quite some [time after cadiation has been delivered. At this 2 month interval in this particular aries the 5 pe animals showed a moderate increase in thyroid weight suggesting that cell division aad actually taken place. The 50 pic animils on the other hand showed essentiBlly no aypertrophy of the gland. Still later in the post radiation period the capadity for 2xcessive labeling could not be produced in the 5 pe group of animals. At this time, these thyroids did not increase appreciably in weight. Even after a longer rpcovery period (8 months) following the 1I the extreme degrees of labeling could rpt be sreduced with thiouracil in the 5 yc animals. These observations are illustrpted in Tigure 9. It was of special interest to find that the large bizarre nuclear forms , l wreparing to divide usually become enlarged, but here the abnormally large n vyhich presumably have not divided, were making more DNA.