The clinico-pathologic changes in beagles after a single inhalation exposure to about 2 mc of Pu239o2 aerosol are described. Death occurred in 4 of 5 dogs within 96 days. The earliest clinical change was a progressive lymphopenia beginning within 1 week after exposure. Pulmonary radiation damage was evidenced by increased respiratory rates 5 weeks after exposure. Respiratory rates of over 200 per minute, a tenfold increase above the normal breathing rate, occurred before death. Cyanosis, was an additional indication of injury to the lungs. Other important signs included anorexia, dehydration, and progressive weight loss of up to 25% of pre-exposure values. Tissue changes were limited to the lungs and associated lymph nodes which contained 99% of the body burden of plutonium.