Deficiency of the trace element selenium causes disease in domestic animals and may also be implicated in the pathogenesis of some human illness. In this study, the triple-lumen perfusion method was used to measure the rate of absorption of trace quantities of selenium (50 micrograms/liter in a physiological electrolyte solution) from the jejunum when given as D,L-selenomethione, D,L-selenocystine, or sodium selenite to healthy dogs in vivo. Selenium absorption from the test segment (expressed as percent administered dose per centimeter +/- SEM) was 1.97 +/- 0.04 from D,L-selenomethionine, 1.15 +/- 0.06 from D,L-selenocystine, and 0.51 +/- 0.07 from sodium selenite (P less than 0.01, N = 5). In separate studies in four anesthetized dogs, the jejunum was perfused with L-[75Se] selenomethionine while concentrations of 75Se were measured in the portal venous blood; these studies established that [75Se]selenomethionine disappearing from the gut lumen corresponded quantitatively to 75Se appearing in the portal venous effluent (74 +/- 6%) and incorporated into intestinal tissue (24 +/- 5%). These results are consistent with the hypothesis that the absorption of amino acid-bound selenium is accelerated by the specific amino acid active transport mechanisms in the gut mucosa. Sodium selenite is absorbed more slowly, possibly by simple diffusion through the intestinal mucosa, than the amino acid-bound selenium compounds.