Beagle dogs are currently being exposed to 239PuO2 aerosols as part of a low-level effects study which will include both 238Pu and 239Pu. Desired alveolar burdens range from 2 nCi to 3 μCi, divided into 6 groups. Respiration parameters measured during exposure include total inspired volume (VOL), respiratory frequency (RR) and tidal volume (TV). Aerosol parameters measured are concentration (CONC) and the aerodynamic equivalent size distribution, characterized by the activity median aerodynamic diameter (AMAD) and geometric standard deviation (GSD). These latter two measures were combined into one parameter, percentage activity less than 0.5 μm aerodynamic equivalent diameter (LS 0.5), to describe the aerosol size distribution. Alveolar deposition (NCI) was determined from thorax counts 14 days post exposure. Percentage alveolar deposition, defined as DEP = (100 × NCI)/(VOL × CONC), varied from 0 to 54 for 36 exposed dogs. A correlation matrix involving aerosol, respiration and physiologic (WT) parameters was computed to determine those factors which most significantly influenced DEP. Subsequent stepwise regression analysis yielded the equation DEP = 3.11 LS 0.5 + 0.015 TV, which accounted for 74% of the variability in DEP. Unlike tidal volume, respiration rate was not significantly correlated with DEP. AMAD was found to be highly correlated with LOG10 CONC (R = 0.895) while GSD was significantly correlated with AMAD (R = -0.597). This suggested that the pre-exposure value of CONG could be used indirectly to compute LS 0.5. Since an estimate of TV could also be obtained during the exposure, DEP values could be estimated from the suggested regression equation, and the volume (VOL) that a dog should inhale to obtain the required alveolar burden (NCI) could be calculated. The results obtained using this procedure are discussed.