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Selenium is an essential trace element found in vegetables as selenomethionine (SeMet) and methylselenocysteine (MeSeCys). In the present study, we used stable isotopes of Se to investigate differences between how SeMet and MeSeCys are metabolized, using methylseleninic acid (MSA) as a reference methylselenol source. A mixture containing 76Se-SeMet, 77Se-MeSeCys, and 82Se-MSA (each 25 μg Se/kg b.w.) was orally administered to rats, and then, speciation analyses of Se in urine and exhaled gas were conducted using HPLC-inductively coupled plasma (ICP)-MS and GC-ICP-MS, respectively. The proportions of 76Se-, 77Se-, and 82Se-selenosugar (Se-sugar) to total urinary Se metabolites originating from each tracer were very similar, while the proportion of 77Se-tirmethylselenonium (TMSe) was much less than that of76Se- and 82Se-TMSe in urine, suggesting that77Se-SeMet is less efficiently metabolized to TMSe. Similarly, there was significantly less 77Se-dimethylselenide (DMSe) originating from 77Se-SeMet than76Se- and 82Se-DMSe originating from 76Se-MeSeCys and 82Se-MSA in exhaled gas. It is generally accepted that DMSe and TMSe are metabolites of methylselenol, a putative metabolic intermediate in Se metabolism. Methylselenol is believed to be responsible for the cancer chemoprevention effects of Se. These results suggest that MeSeCys is converted to methylselenol more efficiently than is SeMet and that urinary TMSe and exhaled DMSe might be useful biomarkers for the generation of cancer chemopreventive forms of Se. [ABSTRACT FROM AUTHOR]