Inhalation studies were undertaken in which plutonium dioxide (/sup 239/PuO/sub 2/) was administered to either unanesthetized Wistar rats or anaesthetized baboons. In both groups of animals some deaths occurred from acute lung damage resulting from cell necrosis particularly to vascular tissue followed by alveolar oedema. At later stages, marked interstitial pneumonitis and interstitial fibrosis occurred and deaths resulted from respiratory insufficiency preceded by high arterial blood pCO/sub 2/ and low pO/sub 2/. In rats as many as 50% of the animals finally developed lung neoplasms but only two such tumors were found in baboons. Attempts were made to correlate biochemical parameters with observed tissue damage and animal mortality.