To study the long-term translocation and biological effects of inhaled plutonium, 40 beagle dogs were given single 10–30 min exposures to 239PuO2 aerosols. Thus far eight dogs have died 28–48 months post-exposure. The body burdens at death ranged from 1–3 microc, with about 50 per cent of the body burden in the lungs, and as much as 50 per cent in the bronchial and mediastinal lymph nodes. The liver contained 2–10 per cent, and the skeleton, 1–4 per cent. Cardiopulmonary insufficiency and lymphopenia were the primary clinical signs. Pathologic changes included fibrotic and metaplastic changes in the lungs, and fibrosis of the bronchial lymyh nodes. In addition, pulmonary neoplasia was detected in four dogs.