histological patterns of those neoplasms produced by soitrogens along, fbr by 121; alone, or by a combination of the two. Labelling of nuclei with 3H-thyhi usually more abundant in neoplasms than in the extrancdular tissue. I frequency of labelling is more often seen in areas where normal follic given way to minute microfollicles or solid cellular areas between fo preliminary impression is gained that these areas represent the origin or actual neoplasms. As part of a background study of this investigation, it has seeme to obtain information on the capacity of normal thyroids et different respond with mitotic activity when challenged with the acute administr antithyroid drug. It is also important to determine the frequency of 2 normal animals on a standard iodine intake. Although the data are not complete cr ready for presentation, it is now clear from preliminary refi autoradiographs that in the very young growing rat, labelling of nucleiji and that, as the age of the animal progresses, fewer and fewer cells spentaneous mitosis. In very old animals it is rare to find even an oc labelled nucleus indicating mitosis. Ina similar way, the ability of respond to the acute challenge of antithyroid drug is very vigorous in animal. In the animals later in the life span, the response in the f nuclei is still clearly evident, but far less vigcrous. assembled from control animals in these studies. of labelled This informati nis being Efforts have been made to acquire information on time required forfmitosis to take place in the normal thyroid and the degree to which the steps of mitosis may be affected by radiation. Data which we have previously published undef this contract show that following radiation, DNA builds up in nuclei in prez division, but that actual division of the cell may be thwarted. The re very large nucleus which contains double or several times the amount of DNA customarily found in the normal nucleus, as if the cell had attempted t# go through mitosis several times. In pilot_experiments with autoradiographs, it appeared that following a single injection of 3H-+ thymidine all labelling of miclei wa completed by four hours. Subsequent work of others has suggested that labellirg complete in one hour. In recent preliminary experiments, we have injected a large group of identical young rats with $H-thymidine and sacrificed them at 15 minutefintervals in an effort to demonstrate when the rising incidence of labelled nuclei ches 2 plateau. The plateau should indicate that no more labelled cells are sfill dividing. A plateau of labelling of nuclei was found, but it occurred later than @xpected. Data on paired nuclei was difficult to interpret. Although only those nucle which are preparing for division take up 3H-thymidine and the autoradiographs arefbeautifully imprinted from the isotope localized in individual nuclei, the geometryland range of. the beta ray in this preparation poses problems in quantitation. This @xperiment is to be repeated soon with simultaneous observations on the 34-thymidine the bicod, its disappearance from the blocd and the appearance of its metabolic degradaticn products. Information gained may be useful in determining the effect og radiation on the duration of the process of mitosis in radiated thyroids. OBSERVATIONS ON THE DEVELOPMENT OF THYROID NODULES IN POPULATIONS ESPOSED RADIOIODINE FALLOUT AS THEY RELATE TO STUDIES UNDER THIS CONTRACT (MARSHALLESE ) -—— (UTAH-NEVADA) The Marshallese Study In February and March of 1969, the responsible investigator spent Jive weeks| in the Marshall Islands participating in the annual review cof thyroids ¢@f the