The composition of the fission-product mixture in the various tissues (with the ‘exception of the thyroid) appears to be dominated by one or a group of fission products, as seen from the similarity in the radioactive decay rates:for all the tissues during the first4'days, The thyroid, of course, has an affinity for iodine’®! and the shorter -lived iodine wwadio- - isotopes. “+ Lo; f PA gamma spectrum analysis will be used in future experiments to identify the major fission-product components of the simulant material, ‘ ’ * 7 Following the administration of the simulant material by stomach tube, the radioactive decay of the material deposited in the various tissues | (Fig. 3B) was very similar to that. observed following the inhalation ex- _posure, The basis for the similarity probablyilies in the fact that’ in: Ree either mode of administration a large fraction“of the radioactivematerial Mw enters into the systemic circulation by the Gl/portal of entry. ES Biological Loss of Simulant The gammaactivity of each tissue taken iromimice sacrificed/at various time intervals following exposure was corrected for radjoactive decay back to | hr after exposure by the use of the above-mentioned radioactive decay curves, The resultant curve approximately describes the biological! loss of the fallout simulant in that tissue (Fig. 4). The activity of the GI tract and its contents decreased the most rapidly, = due to the excretory nature of this organ. As previously noted,the hio- logical decay of the radioactive material in the GJ tract was veryrapid and could be separated into two components, The initial rapid loss of material with a few hour's half-life probably corresponds to the rapid loss of material from the Gl tract via excretion, The second component with a half-life of several days was probably related to the slower pulmonary elimination of the lung "fixed" activity and to the normal excre= tion of internally deposited material, The rate constant for the‘loss of activity from the blood is also quite high initially, due to the rapid exchange of matcrial from blood to other tissues, After this initial: rapid loss, the activity in the blood approaches a constantlevel, Following administration of the simulant by gavage, the material rapidly passed through the GJ tract at approximately the same rate that was observed following inhalation, The rate of loss of the simulant through the GI tract was such that the stomach and small intestine contained | per cent of the maximum activity at 2 and 5 days, respectively, post -radiation, The activity in the large intestine fell to | per cent of the original activity by the 18th day. » ‘ -10UNCLASSIFIbD are eam ee