[en] The effectiveness of a single dose of either Ca-DTPA or Zn-DTPA (diethylenetriaminepentaacetate) in removing intravenously injected ²⁵²Cf and ²⁴¹Am from the main organs of retention in the rat was found to depend on the dosage of the chelates (10-1000 μmolkg^-1 i.p.) and on the time interval between radionuclide injection and DTPA administration. The efficacy of Zn-DTPA after 1.5 min was considerably less than that of Ca-DTPA over the whole dose range, whereas there was no important difference after 24 h. The ratio of equally effective Zn-DTPA: Ca-DTPA doses was between 5 and 10 after early treatment and approximately 1 after 24 h. Chelate effectiveness was markedly less dependent on dose after 24 h than after 1.5 min. The results of a study on the influence of time (1.5 min - 64 d) on the effectiveness of 30 μmolkg^-1 of either chelate indicated a rapid initial loss of efficacy and confirmed the distinctly higher effectiveness of Ca-DTPA than Zn-DTPA during the first 24 h. In further experiments different schedules were examined for long-term treatment of ²⁵²Cf or ²⁴¹Am incorporation. If treatment was initiated after 1.5 min, it was less effective with only Zn-DTPA than with Ca-DTPA; if the first dose was Ca-DTPA and the following doses were Zn-DTPA, the total removal was the same as in the series of Ca-DTPA doses. Both chelates were virtually equally effective when treatment started on the fourth day after radionuclide injection. The results indicate that Zn-DTPA can be used instead of Ca-DTPA for long-term treatment of internal contamination with trivalent actinides. In acute emergencies, however, Ca-DTPA should be preferred. The results for ²⁴¹Am and ²⁵²Cf were similar qualitatively. However, ²⁴¹Am was mobilized to a lesser extent than ²⁵²Cf, especially during the first 24 h after radionuclide injection. (author)