The relative physiological and toxicological properties of americium and plutonium have been studied following their intravenous administration to rats. The urinary and fecal excretion of americium was similar to that of plutonium administered as Pu(N03)4. The deposition of americium the tissues and organs of the rat was also similar to that observed for plutonium. The liver and the skeleton were the major sites of deposition. Zirconium citrate administered 15 minutes after injection of americium increased the urinary excretion of americium and decreased the amount found in the liver and the skeleton at 4 and 16 days. LD305° studies showed americium was slightly less toxic when given in the acute toxic range than was plutonium. The difference was, however, too slight to be important in establishing a larger tolerance dose for americium. Survival studies, hematological observations, bone marrow observations, comparison of tumor incidence and incidences of skeletal abnormalities indicated that americium and plutonium have essentially the same chronic toxicity when given on an equal basis. These studies support the conclusion that the tolerance values for americium should be essentially the same as those for plutonium.