[en] Syrian hamsters have been exposed via inhalation to polydisperse aerosols, or one of three sizes of monodisperse aerosols of ²³⁸PuO₂ to achieve seven levels of initial lung burden in the polydisperse study and four levels at each of the three monodisperse studies. Animals were exposed as young adults at 84 days of age. Groups of animals are being maintained for serial sacrifice to determine the radiation dose pattern and for lifespan observation to determine dose-response relationships. The data show slight translocation of ²³⁸Pu from the lung to liver and skeleton to 256 days post-exposure, confirming the relatively insoluble nature of the ²³⁸PuO₂ particles. Significant excess mortality has been observed in animals exposed at the higher levels (100 to 720 nCi achieved initial lung burden) in the polydisperse study. The major pathological finding in animals that have been examined has been radiation pneumonitis and pulmonary fibrosis. No malignant lesions have been observed to date. Observations are continuing on all survivors. (U.S.)