Although the flavonol quercetin is used as a supplement in commercial dog food, data on quercetin bioavailability in dogs are not available. Thus, we investigated quercetin bioavailability (measured as area under the concentration-time curve) in nine adult beagle dogs at an oral dose of 10 mg/kg body weight (b.w.). The major fraction (>80 %) of flavonols circulating in blood plasma were conjugated metabolites of quercetin. The absolute bioavailability of quercetin (i.e. the fraction that reaches the systemic circulation) was only about 4 %. We also compared the oral bioavailability between the aglycone quercetin and its more often used glucorhamnoside (rutin) and 3-O-glucoside (isoquercitrin) at an equimolar dose of 30 mumol/kg b.w. (corresponding to 10 mg quercetin/kg). Quercetin and isoquercitrin were mainly absorbed in the small intestine with isoquercitrin being one and a half times more bioavailable than quercetin. Maximal plasma concentration after isoquercitrin treatment was 0.89 (sem 0.07) mumol/l. Although quercetin absorption from rutin was delayed, relative bioavailability was not lower than from the aglycone itself. The latter observation is in clear contrast to findings in human subjects, pigs or rats and might indicate that rutin is a better source of quercetin in dogs than in other species. However, potential in vivo quercetin effects beyond the gastrointestinal tract are limited by the intensive metabolism as well as by the rather low bioavailability of this flavonol.