An inhalation study of plutonium dioxide in twenty-two dogs following single exposures is described. The study emphasized the clearance of PuO2 dust from the lungs and its fate. The pulmonary clearance process, computed by several methods including in vivo counting, can be described as a bi-phasic exponential with mean biological half times of about 1 and 400 days, respectively. By excluding the first post-exposure week, the lung clearance can also be approximated by the power function t−0.22. The tissue distribution of plutonium in animals sacrificed between 16 and 468 days after exposure is described. The bronchial lymph nodes and the lungs had the highest plutonium concentration and, on the average, accounted for 95 per cent of the body burden. The principal pathological effects noted were associated with these tissues. Few extrapulmonary tissues, except the gonads and lymph nodes, appear to have received important radiation doses. The absorption of plutonium averaged 3.5 per cent of the PuO2 deposited. A disproportionate part of the absorption appeared to have occurred shortly after exposure. The report also describes the elimination kinetics of plutonium and discusses some problems in interpreting excretion data.