me ne owned CAPELUMIICS, 2 Small trace dose of 4+91I in addition to the Areated thymidine bas been given to the animals four hoursbefore sa ioe. Autoradiographs prce-ured from microscopic sections promptly after the rimove of the thyroid ser:« <o identify any local areas in the gland where 131 utilization is dit’..ont from the rest of the gland. Because of the shért half life of 131I, this -s0tope disappears from the microscopic sections after two or three months. iow autoradiographs then prepared from other microsce acent to those used to demonstrate 1911,will show only the location df tiated thymidine. Subsequent comparison of the two autoradiographs microscopic sections,which are almost identical, reveals two types of about the same cluster of cells which appear in both preparations. the from amatic We have several large groups of rats whose thyroids have been Others are stimulated acutely for a few days berore ice. Still remain unstimulated, except for the intrinsic stimulus which the animal fy receive as a result of diminution in hormone by the radiated thyrpid. oe sporvtanmouslYe ALthough ohthe theincidence’ of necplasms saees Snood byLAT act SeecigdfieantTY om» han lesions have bean found their present more ieant. Rats weighing 100-130 grams were fi-st used with ultimate poor survival, owing to an epidemic of pulmonary infection; howsver, the expected bizarge nuclear forms and some giant nuclei, which had previously beer shown to contain noes ive amounts of chromatin, did appear. labeled thymidine. Many of the large nuclear forms took gp the More recently, weanlings were prepared by giving doses of 1311 from|2.5 microcurie to 40 microcurie doses. Each ret has been identified and its individual uptake of the dose determined so that the individual amourt o: radiation can be more precisely known. Tus far, autoradiogrephs of rat jt oad rediated ten months or less before sacrifice have been reviewed. As we lve previously observed, hypertrophy of the gland may be produced by goitre for quite some time after the 1321 is given, but after a considerable lag Cong gurge of mitotic activity after several sonths cf recovery, as we had p osuly described in an earlier series, is confirmed. ‘<t is apparerrtly at this time that hypertrophy of the gland can be produced even though it carmot be produce! later.