-20- the same rate until the 200th day. The eerly high levels may be due to contamination of the surface of the gills and possibly to excretion of salts through the gills. From the tenth day on, the pettern of decline of the gill is the same as that of muscle. The activity level was generally higher in the gill than in the muscle by less than a factor of two on a wet weight basis. No chemical analyses of gill tissue were made. Discussion During the first 150 days following a nuclear detonation the rate of decline of radioactivity in organisms on atoll islands may be considered to approximate the rate of decay of mixed fission products. This conclusion 1s supported further by data from collections at Rongelap Atoll in 19542729 Errors in the estimate of future levels based on this approximetion would tend toward the prediction of higher levels than would actually be attained in the first 150 days. The wide spectrum of avail- able radionuclides present in the early period following a det- onation may be available to individual organisms in extremely minute amounts; consequently, differences in the rate of decline reflecting selectivity by an organism are masked, since various combinations of the short lived nuclides could result in DOE ARCHIVES approximation of mixed fission products decay. The availability of a wide spectrum of radionuclides during the first few days might be due not only to the presence of these nuclides, but also to the fact that they could potentially be absorbed directly V6