We evaluated the effect of chemical form and feeding regimen on the gastrointestinal (GI) absorption of plutonium in adult mice at plutonium concentrations relevant to the establishment of drinking water standards. Mean fractional GI absorption values in fasted adult mice were: Pu(VI) bicarbonate, 15 x 10/sup -4/; Pu(IV) bicarbonate, 20 x 10/sup -4/; Pu(IV) nitrate (pH2), 17 x 10/sup -4/; Pu(IV) citrate, 24 x 10/sup -4/; and Pu(IV) polymer, 3 x 10/sup -4/. Values in fed adult mice were: Pu(VI) bicarbonate, 1.4 x 10/sup -4/; Pu(IV) polymer, 0.3 x 10/sup -4/. Pu(VI) is the oxidation state in chlorinated drinking waters and Pu(IV) is the oxidation state in many untreated natural waters. To assess the validity of extrapolating data from mice to humans, we also determined the GI absorption of Pu(VI) bicarbonate in adult baboons with a dual-isotope method that does not require animal sacrifice. Fractional GI absorption values obtained by this method were 23 +- 10 x 10/sup -4/ for fasted baboons (n=5) and 1.4 +- 0.9 x 10/sup -4/ for fed baboons (n=3). We have so far validated this method in one baboon and are currently completing validation in two additional animals. At low plutonium concentrations, plutonium oxidation statemore » (Pu(VI) vs Pu(IV)) and administration medium (bicarbonate vs nitrate vs citrate) had little effect on the GI absorption of plutonium in mice. Formation of Pu(IV) polymers and animal feeding decreased the GI absorption of plutonium 5- to 10-fold. The GI absorption of Pu(VI) bicarbonate in both fed and fasted adult baboons appeared to be the same as in fed and fasted adult mice, respectively. 17 refs., 2 tabs.« less