[en] Studies to determine the effects of age at exposure on metabolism, dosimetry and biological effects of inhaled particles of ²³⁹PuO₂ have been initiated in aged beagle dogs (8.0 to 10.5 years of age at exposure). Beagle dogs have been exposed to 1.5 μm AD particles of ²³⁹PuO₂ resulting in initial lung burdens ranging from 0.03 to 0.76 μCi/kg body weight. Dogs exposed to the aerosol diluent serve as controls. There were four blocks exposed in the past year, one of males and three of female dogs. Nine dogs died during the year. Seven of these had radiation pneumonitis and two died of nonradiation induced diseases. The surviving dogs are as long as 538 days after exposure. There are three blocks of male dogs planned for exposure in the next 12 months