The efficacy of CaDTPA and ZnDTPA, the chelating agents of choice for several actinide elements, have been evaluated after the deposition of thorium in the rat lung in widely different amounts. The results showed that: 1 When the initial mass concentration of thorium simulated human exposure to four times the annual limits on intake for 232Th, the prompt (300 or 1000 μmol kg −1 body weight at 0.02 d) or repeated (30 or 300 μmol kg−1 body weight at 0.02, 0.25, 1,2,3 d) administration of CaDTPA were at best only moderately successful for enhancing the elimination of thorium. By 7 d after exposure, the body contents of thorium were, respectively, about 74%, 65%, 90% and 74% of those present in untreated animals. 2 When the mass concentration simulated 1.7 × 10-3 times the annual limits on intake for 232Th, the efficacy of treatment was not increased appreciably despite the substantial reduction in mass. After the repeated administration of CaDTPA at doses of 30 and 300 μmol kg-1 using the protocol above, the body contents of thorium by 7 d were, respectively, 69% and 51% of those in untreated animals. 3 Under comparable conditions, the efficacy of ZnDTPA was less than CaDTPA. The results suggest that more effective chelating agents are needed for the treatment of workers exposed to water soluble thorium compounds.