SCOPE: Benzoxazinoids, which are natural compounds recently identified in mature whole grain cereals and bakery products, have been suggested to have a range of pharmacological properties and health-protecting effects. There are no published reports concerned with the absorption and metabolism of bioactive benzoxazinoids in humans. METHODS AND RESULTS: The absorption, metabolism, and excretion of ten different dietary benzoxazinoids were examined by LC-MS/MS by analyzing plasma and urine from 20 healthy human volunteers after daily intake of 143 μmol of total benzoxazinoids from rye bread and rye buns. The results showed that 2-β-d-glucopyranosyloxy-1,4-benzoxazin-3-one (HBOA-Glc) and its oxidized analog, 2-β-d-glucopyranosyloxy-4-hydroxy-1,4-benzoxazin-3-one (DIBOA-Glc), were the major circulating benzoxazinoids. After consuming a benzoxazinoid diet for 1 week, morning urine contained eight benzoxazinoids with abundant HBOA-Glc (219 nmol נμmol(-1) of creatinine). The sulfate and glucuronide conjugates of 2-hydroxy-1,4-benzoxazin-3-one (HBOA) and 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) were detected in plasma and urine, indicating substantial phase II metabolism. Direct absorption of lactam glycosides, the reduction of hydroxamic acid glycosides, glucuronidation, and sulfation were the main mechanisms of the absorption and metabolism of benzoxazinoids. CONCLUSION: These results indicate that following ingestion in healthy humans, a range of unmetabolized bioactive dietary benzoxazinoids and their sulfate and glucuronide conjugates appear in circulation and urine.