It has been shown that the seleno-bis (S-glutathionyl) arsinium ion, [(GS)(2)AsSe](-), is the major arsenic and selenium excretory product in bile of rabbits treated with arsenite and selenite [Gailer, J., Madden, S., Buttigieg, G. A., Denton, M. B., and Younis, H. S. (2002) Appl. Organomet. Chem. 16, 72-75]. To investigate the in vivo interaction between the other environmentally common oxy-anions of arsenic and selenium in mammals, we have intravenously injected rabbits with different combinations of the arsenic and selenium oxo-anions (arsenite + selenate, arsenate + selenite, and arsenate + selenate) and analyzed the collected bile and whole blood samples by X-ray absorption spectroscopy. Only the injection of arsenite and selenate led to the biliary excretion of [(GS)(2)AsSe](-) within 25 min. Whole blood collected from these animals (25 min postinjection) contained predominantly unchanged selenate, which suggests the presence of a mammalian selenate reductase in the liver. The lack of any significant biliary excretion of [(GS)(2)AsSe](-) in the other treatment groups implies that arsenate was not reduced in the liver on the time scale of our experiments. The relevance of these results for the human toxicology of arsenic and selenium is discussed.