The effects of inorganic (HgCl2) and organic (CH3HgCl) mercury on the intestinal absorption of Se compounds [Na2(75)SeO3, Na2(75)SeO4, L-[75Se]methionine ([75Se]Met)] were determined in 3-wk-old White Leghorn cockerels by the in vivo ligated duodenal loop procedure. The intraduodenal dose contained 0.05 microCi 75Se, 0.01 mM Se, 150 mM NaCl and 0-1.0 mM Hg. In the presence of 1 mM inorganic Hg in the intraduodenal dose, the absorption of the inorganic 75Se compounds was only about 65% of that in the control group, whereas only a slight inhibitory effect on [75Se]Met absorption was observed. Methylmercury had no effect on [75Se]selenite absorption. Precipitation of the 75Se-selenite in the intestinal lumen partly explained the direct interaction between inorganic Hg and Se compounds. Absorption of [75Se]Met and [75Se]selenite was also determined in chicks fed after hatching a purified diet supplemented with varying amounts of Hg (0-500 mg/kg) and Se (0-4 mg/kg). Dietary Hg significantly reduced the transfer of [75Se]selenite to body by enhancing the accumulation of the isotope in the intestinal tissue. Dietary Hg did not affect the absorption of [75Se]Met, but altered the whole-body distribution of this Se compound. Because interaction between Se and Hg was observed mainly between the inorganic compounds and with use of a manyfold excess of Hg over Se, the data suggest that intestinal interaction between these metals is not of great nutritional importance.