Plutonium-238, an alpha-emitting radionuclide, is used as a heat source in thermoeleltric power generators such as have been employed on lunar expeditions of communications satellites and in cardiac pacemakers. It has an 86.4 year half-life and emits 5.5 MeV alpha particles. Beagle dogs were given single 10-30 minute exposures to 238PuO2 aerosols to study the long-term translocation of plutonium and biological effects. Dogs with a terminal body burden ranging from 7-260 muCi were euthanized due to respiratory insufficiency related to plutonium-induced pneumonitis during the first 3 years after exposure. Nine of the 11 dogs euthanized during the 4-6 year postexposure period had osteosarcomas. The terminal plutonium body burden in the tumor-bearing dogs ranged from 0.5-2.6 muCi with 30-55% of the plutonium in the skeleton. Experiments are in progress to further define the dose-effect relationship of inhaled 238PuO2 and investigate the mechanisms of plutonium-induced neoplasia.