The alveolar burden following nose-only aerosol exposure of rodents to highly toxic radioactive materials depends upon the percentage of the inhaled aerosol that is deposited beyond the animals' ciliated epithelium. The quantity inhaled was estimated from the product of the aerosol concentration (nCi/liter), the exposure time (minutes) and the mean minute volume (liters/min) of the animals. For rats exposed to Pu(NO3)4 and sacrificed within an hour, the mean alveolar deposition was 19.2±9.19% for 36 groups of both 239Pu and 238Pu. No important correlations between any of the measured parameters and this initial percentage deposition were observed, in contrast with the results for 239PuO2 deposition measured in rats sacrificed 24 or 48 hours postexposure. In these, the aerosol activity median aerodynamic diameter (AMAD) was strongly correlated with aerosol concentration and percentage deposition (mean—7.9±3.9% for 32 groups) with AMAD over the range of 1.5 to 4.5µm. The 200-fold lower mass concentration of 238Pu compared with 239Pu for a given activity level may explain the fact that the AMAD was correlated with concentration for 239Pu oxide and nitrate, but not for 238Pu compounds.