The retention of heavy metal compound aerosols in the respiratory tract affects the risk induced by their potential toxicity. In this study the long-term retention in beagle dogs has been measured up to five years after a single inhalation of a monodisperse cobalt compound aerosol. In eight experiments the particle size and the chemical composition are varied. All inhalation experiments are carried out with two dogs, also one intravenous injection experiment for the determination of the liver retention. Parallel to the retention measurements extensive excretion analyses during the first year are carried out in order to study clearance pathways. Using the results of additional metabolism experiments the cobalt concentration data of feces and urine can be evaluated.