The mobility of curium dioxide in the rat after pulmonary intubation has been investigated by administering suspensions containing different particle size ranges of the oxide. A major factor influencing the movement of curium from lungs to blood is the formation of hydroxide or hydrous oxide particles about 0·001 μm in diameter. This process is sufficiently rapid for the lung clearance kinetics of the dioxide to resemble those of a soluble compound more closely than those of an insoluble one. Filtration of 0·001 μm particles through the kidneys results in considerably enhanced excretion of curium relative to administered curium citrate. It is concluded that current metabolic models, which assume that solubility in the lung is a prerequisite for transport in body fluids, do not adequately describe the behaviour of curium from the standpoint of radiological protection.