The mobility of americium dioxide in the rat after pulmonary intubation has been investigated by administering different particle size ranges of fractionated suspensions. The clearance of americium from lungs to blood and urine is governed principally by its ability to form low-molecular-weight complexes, probably hydroxides. Americium is excreted in the urine in greater quantities than observed following the administration of americium citrate. It is concluded that the metabolic behavior of americium dioxide more closely resembles that of a soluble rather than an insoluble compound. Even so, the systemic burden of americium cannot be predicted by applying a fixed factor to cumulative urinary excretion in the early lung clearance phase.