-4"Rate of decline" refers to the rate at which radioactivity is decreasing in a given tissue, organ, or organism in its native environment. Levels of activity in the crab tissues three days after the Nectar test ranged from 5 x 10© d/m/g in the gut to 7 x 104 a/m/g in the muscle (Figs. 1 and 2). The rate of decline of activity decreased with time and was different for each tissue, but in general followed the same trend as the decay of mixed fission products during the first 200 days. Thereafter the rate of de- cline for each of the crab tissues approached a constant value with a half life in excess of 20 years. This half life is dependent on factors which include rela- tive abundance and availability of radionuclides in the food and/ or environment, rate of decay of radionuclides absorbed, biological half life and selective uptake of radionuclides. these, except the rate of physical decay, Each of is in turn dependent on varying environmental and physiological conditions. The terms “ecological half life of radioactivity," or more briefly, "ecologi- cal half life" and "rate of decline" will be used to include these factors. Ecological half life will be used as the time required for an organism, or its tissues or organs, in its native environment to lose 50 per cent of its radioactivity. When the ecologi- cal half life and physical half life are equivalent (rate of decline= rate of decay), the tissue in question must be at equilibrium with respect to the radioisotopes it contains. For single isotopes an ecological half life greater than the physical half life (rate of decline <rate of decay) indicates accumulation mrt .