A multitiered inhalation exposure chamber was evaluated for use in aerosol toxicity studies by determining the uniformity of pulmonary deposition in 144 rats simultaneously exposed to 99mTc-CsCl aerosols. The activity median aerodynamic diameter and geometric standard deviation were 1.7-2.1 micron and 1.8 respectively. In one experiment, lung deposition of 99mTc in male and female rats was 728 and 544 nCi, respectively, after 130 min exposure to an aerosol concentration of 173 nCi/L. Analysis of variance revealed a significant effect of several factors on lung deposition. Animals housed on one side of the chamber had lung burdens 8-11% greater than those on the opposite side. Because an animal's location within the chamber had a slight effect on its lung burden of inhaled aerosol we recommend the rotation of animals among the chamber's six tiers during chronic aerosol toxicity studies. The overall coefficient of variation in lung burden was only 21% which is less than the variability reported in small rodents given a nose-only aerosol exposure.