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Beagle dogs were given single exposures to one of seven P U~~~O , aerosols that differed in particle size from 0.2 to 7.6 , u mass median diameter (MMD). Translocation of plutonium to other tissues and excretion in both urine and feces up to a month after exposure were greatest for dogs exposed to aerosols with the smallest median diameter. Dogs exposed to an aerosol with a MMD of 4.3 ,u were studied for about 10 months. The half-time for retention of Pu239 in lungs was about 300 days; however, during this period there was continuous accumulation of Pu289 in bronchial lymph nodes, which resulted in about a 1500-1800 day half-time for total body retention of plutonium. Inhaled and intravenously injected plutonium nitrate (0.2 N HNO,) were compared in still other dogs. After inhalation, 70per centofthe body burden was in the lungs, 10 per cent in the liver and 15 per cent in bone. After intravenous injection, more than 80 per cent of the body burden was in liver and about 6 per cent each in the spleen and.bone. The rate of excretion in urine was about five times greaterafter inhalation than after intravenous injection. Corresponding differences were observed in the levels of plutonium in blood. The results of these studies emphasize the importance of the chemical form and the particle size of inhaled plutonium aerosols on retention, translocation and excretion, and point out the problems to be encountered in estimating the body burden from excretion analysis.

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