The effect of particle size on the movement and solubility of239Pu02has been investigated in rats by administering fractionated suspensions of a polydisperse aerosol. Particles > 0.025 /j,m remained predominantly in the lungs after intubation or deposited in the liver after intravenous injection. Conversely, particles of about 0.001 fim diameter move rapidly from the lungs to blood. In blood, the particles undergo a rapid reaction. It is postulated from the experimental evidence that this involves citrate ions and that a chemical species of239Pu with citrate, as yet unidentified and referred to as an "intermediate" species, is formed. This then could proceed to form239Pu citrate and eventually239Pu transferrin. The "intermediate" species is the predominant form of239Pu in urine within tens of minutes following an intravenous injection of 0.001 /j,m239Pu02particles but is subsequently transformed to239Pu citrate. There are two implications from this study. One is that a polydisperse aerosol containing a high 0.001-/j,m particle fraction will behave differently to that predicted by the Task Group Lung Clearance Model, which defines239Pu02as an insoluble compound. The other is that any attempt to estimate the body content (excluding lungs) of 239Pu by applying a fixed factor to cumulative urinary excretion would have to be adjusted to account for the enhanced excretion of239Pu (compared with administered239Pu citrate) in the early period after an intake of 0.001-/j,m particles. The intravenous injection of Na3Ca-DTPA effectively removed the 0.001-,m component of a polydisperse aerosol of239Pu02if administered within 1 hr.