Intestinal metabolism of the subtoxic level of selenite in rats was investigated using a double-perfusion system, which is an in situ, in vitro preparation in which the intestinal lumen and its vasculature are perfused simultaneously. The toxicity of sodium selenite was determined by inhibition of 3-O-methyl glucose (3MG) absorption and by histological examination. Levels of 1.2 mM selenite were required to significantly (p<0.05) reduce 3MG intestinal absorption (58+/-11%, mean+/-SD). Cation-exchange chromatography was used to determine the chemical forms of Se from selenite after using luminal concentrations of 1-200 microM in vascular perfusates. The chemical forms were selenite, selenodiglutathione (GS-Se-SG), mixed selenoglutathione plus cysteine (GS-Se-CYS), selenodicysteine (CYS-Se-CYS), protein-bound Se, and unidentified selenocompounds. Selenite was the predominant selenocompound found in vascular perfusate, but protein-bound Se was the predominant metabolite from selenite present in the vascular effluents. There was a corresponding increase of all metabolites with increased levels of selenite with time of absorption, but not with increased concentration of luminal selenite.