-13- Unlike the fishes -- the aquatic vertebrates -- the land vertebrates are limited in both kinds and numbers for the experiments. on the islands of the atolls used Two kinds of birds, the fairy or white tern and the common noddy tern and the insular field rat are sufficiently abundant and have adequate distribution to be useful as study material. The terns gather their food from the sea, where they feed mostly upon small pelagic fish, which in turn feed upon plankton. Radioisotopes not absorbed and retained by the birds may be dropped upon the islands in the resting and nesting areas. This transfer of radioactive materials from the sea to the land by the fish-eating birds is a useful way to measure the reverse flow or "uphill" transfer of minerals from sea to land. An in- direct effect is the change of availability of radioactive materials following fertilization of the soil by the birds. Studies of the rats on the islands near the detonation sites have proven to be extremely useful in evaluating the over-all effects of atomic weapons. Since they are confined to the small islands and must both live and eat on the contaminated areas, they may receive both external as wellas_ in- ternal exposure to radiation. The highest concentration of radioactive materials! in the bone, with liver and kidney somewhat lower. inthe rats is The skin, muscle, lung and intestinal tract are, in general, lowest in radioactive content. The relative levels in the various tissues vary with time, although the