Twenty-seven beagles were individually exposed by inhalation to <tex-math>${}^{144}{\rm CeCl}{}_{3}$</tex-math> in a CsCl vector aerosol (Activity Median Aerodynamic Diameter ∼1.5-2.2 μm) for 4-10 min yielding initial body burdens of <tex-math>$1.6-4.4\ \mu {\rm Ci}\ {}^{144}{\rm Ce}/{\rm kg}$</tex-math> body weight. Of the <tex-math>${}^{144}{\rm Ce}$</tex-math> activity inhaled, an average of 71% (range = 51% to 94%) was deposited in the total body and an average of 35% (range = 13% to 55%) was associated with long-term retention. The behavior of the inhaled <tex-math>${}^{144}{\rm Ce}$</tex-math> was followed by whole-body counting, excreta collections, and tissue analyses following serial sacrifice of groups of three dogs from 2 to 512 days after exposure. After an early clearance from the upper respiratory tract, most of the remaining body burden was retained with an effective half-life similar to the physical half-life of <tex-math>${}^{144}{\rm Ce}$</tex-math> 284 days. As <tex-math>${}^{144}{\rm Ce}$</tex-math> was absorbed from the lung, it was deposited primarily in liver and skeleton; maximum tissue burdens were observed at 8 days after exposure. An analog model describing the metabolic behavior of <tex-math>${}^{144}{\rm Ce}$</tex-math> inhaled in this form by beagles was used to calculate tissue doses. A long-term retained burden of 1.0 μCi <tex-math>${}^{144}{\rm Ce}/{\rm kg}$</tex-math> body weight in a beagle resulted in potential infinite doses to liver, lung, and skeleton of 220, 90, and 67 rads, respectively. Direct extrapolation of these results to humans indicates that the limiting maximum permissible annual intake based on 15 rem/year and a pulmonary deposition of 20% is that for liver, 13 μCi. This results in an average occupational air concentration that is one-half the maximum permissible concentration in air currently recommended for <tex-math>${}^{144}{\rm Ce}$</tex-math> inhaled in a soluble form.

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