Dissolution and transport of particles deposited in the lower respiratory tract (LRT) are important factors when estimating clearance and the long-term hazard associated with inhaled radioactive aerosols. Three methods for the in uitro determination of rates of dissolution in simulated biological fluids of respirable particles are described. Short term an vitro dissolution patterns of 144CeCl3 aerosol particles and various samples of 95Zr-95Nb aerosol particles were determined and compared with data on the in vivo dissolution of similar particles in the LRT of experimental animals. General agreement between the in vitro and in vivo dissolution patterns was observed. The long-term (weeks) in vitro rates of dissolution of narrow size group 90Sr-fused clay particles were measured in static and parallel flow systems. The rate constants of dissolution of 90Sr-fused clay particles in the two systems were calculated as 3.1 × 10−8 g/cm2/day and 3.3 × 10-s g/cm2/day, respectively. The estimated rate constant of dissolution of 90Sr-fused clay particles in the LRT of a Beagle dog, 3.4 × 10−8 g/cm2/day, was in good agreement with the in vitro rate constant of dissolution.