[en] We have initiated a study using rats to determine the modifying effects of pre-existing pulmonary fibrosis on the retention and biological effects of inhaled ²³⁹PuO₂. Pulmonary fibrosis was induced by intratracheal instillation of 8.5 IU/kg body weight of bleomycin at 45 to 49 days before inhalation exposure to an aerosol of ²³⁹PuO₂. The clearance of ²³⁹Pu from the lungs of rats was decreased significantly (p < 0.01) in rats with pre-existing pulmonary fibrosis compared with controls. Respiratory function, lung morphometric measurements and histological evaluations were all consistent with the presence of mild pulmonary fibrosis in the rats treated with bleomycin. Pre-existing pulmonary fibrosis resulted in an increased retention of the initial lung burdens of ²³⁹Pu, apparently by entrapping the particles in fibrotic areas of the lung. The life span of the rats with pulmonary fibrosis was decreased by up to 25% compared with control rats having similar initial lung burdens of ²³⁹Pu. (author)