UNCLASSIFIED exposure, a large amount of material appeared in the G.I. tract. As the phenomenon would not be predicted from the conditions of the experiment. and,further, has interesting implications concerning the route of entry of radioactive material into the body, it was felt that this aspect of the problem merited further attention. The present study attempts to obtain data on the relative importance of the G.I. tract and the respiratory system as portals of entry into the sys - temic circulation for particulate material following an inhalation exposure. EXPERIMENTAL Two prototype fission products were selected for this experiment, re- presenting two extremes of solubility and thus indicating the probable range of response to this important characteristic of an aerosol. The fission products selected were the highly soluble SrCl2 and the very insoluble LaCl3. In addition to the use of the two fission products, two different types of carrier material were selected: a biologically inert kaolin clay particle, and sea water administered in the form of a liquid aerosol. Each of three groups of 48 mice were exposed for 3 hours to one of three radioactive aerosols. The aerosols were (1) a dry particle aerosol consisting of kaolin particles with sr85Cl2 adsorbed on the surface; (2) a dry particle aerosol consisting of clay particles with Lal49¢1 3 adsorbed on the surface; and (3) a liquid aerosol of sr85¢12 in sea water. In addition, the two dry particle aerosols of sr85Clz2 and Lal#0¢) 3 were suspended in H20 and administered to two groups of 24 mice by stomach tube. Anionic solution of sr85c1, was also given to 24 mice by stomachtube. The animals were sacrificed in groups of six, at intervals up to 28 days starting at 0.5 hours after exposure. The distribution of gamma activity of the Sr®5 and Lal#° was determined for the respiratory tract (starting at the larynx) and lungs, and for the liver, tibia, head and G.I tract and contents (from the top of the oesophagus to the anus). The skin was first removed from the animals,and the autopsies were carried out