Ultrafine aggregate 239PuO2 aerosols were produced by the vaporization of a new plutonium chelate at less than 200°C with the subsequent thermal degradation of the vapor at 1150°C. The vaporizable plutonium chelate was prepared with ligand 2,2,6,6 tetramethyl-3,5-heptane dione. The aerosol size was determined by electron microscopy and diffusion battery measurements. A short-term inhalation study extending to 16 days was conducted in rats using an aerosol consisting of primary particles of 9 ± 5 nm diameter and 21 ± 14 particles per aggregate. The whole body and lung deposition were comparatively higher than those seen with micrometer size 239PuO2 particles. Both in vivo and in vitro dissolution data indicated low rates of dissolution, k = 10−10 g cm −2 day−1. The distribution of inhaled 239PuO2 particles in the lung was evaluated using autoradiography. Although ultrafine 239PuO2 particles were deposited in all regions of the lungs, a nonuniform distribution was found at all times from 2 hr to 16 days after exposure.