A comparison of the rate at which levels of activity in organisms of the same species change with the passage of time, herein termed decline, with the rate of physical decay, should indicate changes in avallability of the radioactivity to the orgenism concerned. If decline is more rapid than decay a reduction of activity in the environment beyond that caused solely by physical decay is suggested, and conversely, a steeper decay than decline suggests either an increase in availability in the environment or an accumulation or concentration of radioactivity by the organism. Equality of decay and decline suggests that uptake and excretion of radioisotopes have reached an equilibrium with the environment. It will be shown that cases in which physical decay progressed more rapidly than did the rate of decline over the same period of time were rare or lacking. are RR OHNTA