Five human subjects inhaled a mixture of stable and radioactive mercury vapor for periods of 14 to 24 minutes. The subjects retained an average of 74% of that inhaled. Evidence is submitted to show that the retention occurred almost entirely in the alveoli. For 3 days after exposure, the exhaled breath was passed at intervals through activated charcoal traps for sampling periods of 10 to 35 minutes. The data indicated that an average of 7% of the retained mercury was lost in the expired breath, with a half time of 18 hours. Examination of the subjects in a whole body counter yielded average half times for mercury clearance from different parts of the body as follows: lung, 1.7 days; head, 21 days; kidney region, 64 days; chest, 43 days; and whole body, 58 days.