within the different organs and that the differences between the terns and shore birds were caused by differences in feed- ing habits. Although variations within the tissue or organ samples of birds were great enough to preclude analyses of the radioactive disintegration rates by organ, a few conclusions can be drawn from the available data on birds in light of the find-_ In the latter, ings with the post shot rats. individual vari- ation between samples was small enough that the differences between organs was significant, except for those of the GI tract. In addition to this, all of the post shot rat specimens were collected within a 50-yard square area so that the environmental conditions may be considered identical for practical purposes. The coefficients of variability for the organs of the post shot rats were determined and the results are as follows: Stomach Gut & & conconSkin Muscle Bone Liver tents tents Lung Kidney 9.2 Mean Coefficient of variation in percent 28 94 42. 17. 1.9 43 140. B84. 18. 12. 100. 1.0 24. 3.1 27. The lack of marked variability in activity between the lungs of the six rats probably depends mainly upon the effect of particle size and density as related to deposition within lung tissue. Stokinger et al: working with albino rats found - 7% -