The effect of orally administered chelating agents (DTPA, TTHA, TPHA, DFOA) on injected 239Pu and 241Am was studied in rats. Preliminary experiments with chelates given by stomach tube indicated that TTHA was the most promising agent for reducing retention of 239Pu, but when the chelates were administered at lower concentrations in drinking water, DTPA proved equal or superior to TTHA in reducing the tissue content of 239Pu and 241Am; the calcium- and zinc-chelates of both DTPA and TTHA were equally effective. Prolonged, delayed addition of Zn-DTPA to drinking water (from day 4 to 25 post 239Pu) at various concentrations revealed an exponential dose-effect relationship with respect to the total amount of Zn-DTPA offered, but irrespective of whether it was administered in 9 or 21 fractions. Up to one half and three quarters of the 239Pu could be removed from the skeleton and liver, respectively. In order to achieve an equal effect on 239Pu retention, about 30 times more Zn-DTPA had to be added to drinking water than the amount of Ca-DTPA injected. However, drinking Za-DTPA in amounts only about 3 times as high as those of injected Ca-DTPA proved effective in reducing the tissue contents of 239Pu. There was no evidence of toxicity in rats exposed to up to 30 mmoles Zn-DTPA in drinking water.