Little is known regarding pharmacokinetic (PK) or pharmacodynamic interactions of flavonoids with each other: this is of significance since multiple flavonoids are present in the diet and in dietary supplements. Our objective was to determine the effect of quercetin and (-)-epigallocatechin-3-gallate (EGCG), major flavonoids present in the diet, on the PK and bioavailability of biochanin A, a flavonoid with chemopreventive properties. BCA was administered to rats intravenously (5 mg/kg) or orally (16.67 or 50 mg/kg) with or without concomitant EGCG and quercetin. In vitro studies with the human intestinal Caco-2 and human hepatic HepG2 cell lines were performed to evaluate the effects of quercetin and EGCG on the cellular metabolism of BCA, and studies with human breast cancer MCF-7 cells that overexpress P-glycoprotein or BCRP (MCF-7/ADR and MCF-7/MX100 cells, respectively) or MDCK cells that express MRP2 (MDCK-MRP2) were performed to evaluate the effects of cellular efflux. An HPLC assay was used to determine plasma, urine, and cellular concentrations of BCA and the conjugated metabolites of BCA (following enzymatic hydrolysis). The coadministration of quercetin and EGCG significantly increased the BCA area under the plasma concentration vs time curve (AUC) in rats, after both iv and oral administration of BCA. The AUC of total BCA (unchanged + conjugated) was also increased. The increases in BCA AUC reflected predominantly increased bioavailability; this was true even after iv administration due to an apparent increase in the enterohepatic cycling of BCA. Our findings demonstrate for the first time that the administration of multiple flavonoids results in increased flavonoid bioavailability, as well as a decrease in clearance, potentially due to increased enterohepatic cycling.