About 24% of total inhaled Ru/sup 1//sup 0//sup 6/O/sub 2/ was deposited in lungs of mice. Lung retention of deposited Ru/sup 1//sup 0//sup 6/ was studied with a whole-body monitor. Radiochemical assay of tissues from mice killed at intervals up to 70 weeks after exposure confirmed retention in pulmonary tissues. Of three exponential components of the lung clearance curve the longest biological half life was 230 days for about 2% of the total lung deposition. Translocation to other tissues was also observed. Several tissues including ovaries snd adrenal glands attained greater concentrations, though smaller quantities, than the lungs. The total dose to lung over the 70-week period was estimated to be 4600 rads per mu c of total Ru/sup 1//sup 0//sup 6/ deposited. An MPC/sub (a//sub i//sub r/) for Ru/sup 1//sup 0//sup 6/ O/sub 2/ of 1.3 x 10/sup -//sup 8/ mu c per cm/sup 3/ air for a 40-hr week was derived by assuming lung to be the critical organ and by extrapolating the data from mouse to man on the basis of differences in weight alone, ignoring differences in anatomy and physiology. (auth)