Quercetin, a flavonoid, is an inhibitor of P-glycoprotein-mediated efflux transport, and its oxidative metabolism is catalyzed by CYP enzymes. Thus, it is expected that the pharmacokinetics of both intravenous and oral doxorubicin can be changed by quercetin. The purpose of this study was to investigate the effect of oral quercetin on the bioavailability and pharmacokinetics of orally and intravenously administered doxorubicin in rats. The effects of quercetin on the P-glycoprotein (P-gp) and CYP3A4 activities were also evaluated. Quercetin inhibited CYP3A4 enzyme activity in a concentration-dependent manner with a 50% inhibition concentration ($IC_{50}$) of $1.97{mu}M$. In addition, quercetin significantly enhanced the intracellular accumulation of rhodamine-123 in MCF-7/ADR cells overexpressing P-gp. The pharmacokinetic parameters of doxorubicin were determined in rats after oral (50 mg/kg) or intravenous (10 mg/kg) administration of doxorubicin to rats in the presence and absence of quercetin (0.6, 3 or 15 mg/kg). Compared to control, quercetin significantly (p < 0.05 for 0.6 mg/kg, p < 0.01 for 3 and 15 mg/kg) increased the area under the plasma concentration-time curve ($AUC_{0-{infty}}$, 31.2-136.0% greater) of oral doxorubicin. Quercetin also significantly increased the peak plasma concentration ($C_{max}$) of doxorubicin, while there was no significant change in $T_{max}$ and $T_{1/2}$ of doxorubicin. Consequently, the absolute bioavailability of doxorubicin was increased by quercetin compared to control, and the relative bioavailability of oral doxorubicin was increased by 1.32 to 2.36 fold. In contrast, the pharmacokinetics of intravenous doxorubicin were not affected by quercetin. These results suggest that the quercetin-induced increase in bioavailability of oral doxorubicin can be attributed to enhanced doxorubicin absorption in the gastrointestinal tract via quercetin-induced inhibition of P-gp and reduced first-pass metabolism of doxorubicin due to quercetin-induced inhibition of CYP3A in the small intestine and/or in the liver rather than reduced renal and/or hepatic elimination of doxorubicin. Therefore, it appears that the development of oral doxorubicin preparations is possible, which will be more convenient than the intravenous dosage forms. Therefore, concurrent use of quercetin provides a therapeutic benefit - it increases the bioavailability of doxorubicin administered orally.