The flavonol quercetin, is one of the major flavonoids found in edible plants. The bioavailability of quercetin in humans may be influenced by the food matrix in which it is consumed as well as by its chemical and physical form. The objective of the present study was to investigate the biokinetics of quercetin from quercetin-enriched cereal bars and quercetin powder-filled hard capsules. In a randomised, single-blinded, diet-controlled cross-over study, six healthy women aged 22 -28 years took a single oral dose of approximately 130 mg quercetin equivalents from either quercetin-enriched cereal bars (containing 93·3 % quercetin aglycone plus 6·7 % quercetin-4 0 -glucoside) or quercetin powder-filled hard capsules (100 % quercetin aglycone). Blood samples were drawn before and after quercetin administration over a 24 h period. The concentrations of quercetin and its monomethylated derivatives, isorhamnetin (3 0 -O-methyl quercetin) and tamarixetin (4 0 -O-methyl quercetin), were measured by HPLC with fluorescence detection after plasma enzymatic treatment. The systemic availability as determined by comparing the plasma concentration -time curves of quercetin was found to be five times and the c max values six times higher after ingestion of 130 mg quercetin by quercetin-enriched cereal bars than after ingestion by quercetin capsules. In contrast, t max did not differ significantly between the two treatments. The c max values for isorhamnetin and tamarixetin were four and nine times higher after ingestion of quercetin by quercetin-enriched cereal bars than after ingestion by quercetin capsules. In conclusion, quercetin from quercetin-enriched cereal bars is significantly more bioavailable than from quercetin powder-filled hard capsules.Key words: Quercetin: Bioavailability: Onions: Human studies Quercetin (3,3 0 ,4 0 ,5,7-pentahydroxyflavone) is one of the major flavonoids, ubiquitously distributed in edible plants (1) . Epidemiological studies, together with data from animal models and some clinical trials, may suggest a role of flavonoids in general and of quercetin in particular in the prevention of CVD and other age-related chronic diseases (2) . In plasma, quercetin is found in the form of glucuronidated, sulphated and methylated derivatives (3) . Rich sources of dietary quercetin are onions, unpeeled apples, berries, citrus fruits, tea (Camellia sinensis) and red wine (1) . Daily dietary intake of the flavonol quercetin varies considerably. In Western populations, crude estimates of mean dietary intake appear to be 10 -100 mg/d (1,4) . In some countries, quercetin is available as a dietary supplement with daily doses of 200-1200 mg quercetin (manufacturers' recommendations). In addition, quercetin may be used as a nutraceutical for functional foods within a concentration range of 0·008 -0·5 % or 10 -125 mg/serving (5) . Rather high concentrations of * Corresponding author: S. Egert, fax þ49 228 733217, email s.egert@uni-bonn.de Abbreviations: AUC, area under plasma concentration -time curve; c max , ...