This study was designed to measure the effect of aerosol particle size on the deposition, retention, excretion and translocation of 239Pu inhaled as the dioxide by Beagle dogs. To address these questions, young adult male and female Beagle dogs received single brief inhalation exposures to one of three monodisperse aerosols of 239PuO2 having sizes of 0.72, 1.4 or 2.8 microns activity median aerodynamic diameter (AMAD). Periodic collections of urine and faeces were made for each dog until sacrifice at times ranging from 4 hours to 2 years after exposure. The results indicate long term retention of a substantial percentage of the initial pulmonary burden (IPB), and that the retention was affected by particle size. The percentage of the initial pulmonary lung burden retained in the long term component and its effective half time (TE) were 90 per cent with TE = 680 days for the 0.72 micron AMAD aerosol, 68 per cent with TE = 1400 days for the 1.4 microns AMAD aerosol and 82 per cent with TE = 1800 days for the 2.8 microns AMAD aerosol. The major route of elimination of 239Pu from lung was via the faeces, but significant amounts were also translocated to thoracic lymph nodes (approximately 15 per cent IPB by 2 years). Small amounts were translocated to liver (0.2 per cent IPB) and skeleton (0.1 per cent IPB) by 2 years after exposure. The average alpha-radiation dose to the lung was projected to be twice as large for the 2.8 microns AMAD group as for the 0.72 micron AMAD group at 10 years after exposure.