The effect of calcium diethylenetriaminepentaacetate (CaDTPA) and a siderophore analogue 3,4,3-LI(1,2-HOPO) an decorporation of small and large thorium masses ( 234 Th 46 ng, 234 Th 46 ng + 232 Th 5 μg per rat) instilled into the lungs was studied. Along with repeated injections, chelating agents were administered continuously by mini-osmotic pumps over 14 d. Treatment with CaDTPA alone was partly successful in the decorporation of both thorium masses. Greater decorporation was achieved with 3,4,3-LI(1.2-HOPO) in pumps and CaDTPA in injections. In the whole body, lungs and skeleton, 23%, 24% and 24% of control radioactivity was found, respectively, for the small mass of thorium. When the large mass of thorium was instilled, only continuous infusion of 3,4,3-LI(1.2-HOPO) at low concentration efficiently prevented transfer of thorium to the skeleton (reduced to 9% of control). Both the whole body and lungs retained 50% of control radioactivity.