Wistar rats were given a single inhalation exposure to high-fired /sup 241/AmO/sub 2/ particles and examined over their life span. A total of 310 rats were used: 259 exposed to /sup 241/Am for life-span study, 30 exposed to /sup 241/Am for early metabolism study, and 21 unexposed life-span controls. The activity median aerodynamic diameter of the aerosols was 0.75-1.39 ..mu..m. About 55% of alveolarly deposited /sup 241/Am was cleared from the lung with a half-life of 0.5 days, 37% with a half-life of 7 days, and 8% with a half-life of 580 days. Group mean lung doses ranged from <5.7 rad up to 1500 rad. Significant early mortality due to radiation pneumonitis was seen only in the highest exposure group. The percentage of rats with lung tumors was 0% for controls (21 rats), 1% at lifetime lung doses <10 rad (139 rats), 7% at 10-50 rad (86 rats), 0% at 50-100 rad (9 rats), 60% at 100-500 rad (10 rats), and 7% at 500 rad (15 rats). Only one liver and one bone tumor were found in all exposed rats, both at lifetime tissue doses <10 rad. The fate and carcinogenicity of inhalated /sup 241/AmO/sub 2/ in the lung ofmore » rats were similar to what has previously been described for inhaled /sup 244/CmO/sub 2/.« less