that particle size greatly affected the amount of deposition in the areolar spaces with increases of as much as 10-fold with a reduction of mass-median diameter from 2.6 to .45 nn. Taplin et al.? found that in rats lung retention of particles with a mean size ofn~-lp was strongly dependent upon density of the particles. Autoradiographs of lungs of rats collected for the present work indicate a diffuse deposition of the radioactive material within the lungs except for the bronchii, where the activity is more concentrated and irregular (Fig. 13). The results found in the autoradiographs as well as the lack of appreciable variation in samples may well be dependent upon the factors of selection and retention of particles by size and density, especially since the particles retained by the lungs are of & small mean diameter and are more nearly the density of the heavier Nevada sand and BaSo, particles than the “aye particles Taplin found to be retained to @ greater degree in lungs of rats. The least variability in organs and tissues of the post shot rats was found in the liver, the variability of kidney and lung being slightly greater. When the average values for each organ or tissue of the post shot rats are compared, muscle is the lowest and bone the highest in activity of the samples taken. however, Lung tissue, is almost as low as that of muscle tissue. In the birds from Rigili, Rojoa, Aaraanbiru, 7 wae and Runit