This study aimed to investigate the inhibition effect of flavonoids on monocarboxylate transporter 1 (MCT1) in Caco-2 cells. The cellular uptake of benzoic acid was examined in the presence and the absence of naringin, naringenin, morin, silybin and quercetin in Caco-2 cells. All the tested flavonoids except naringin significantly inhibited (P<0.05) the cellular uptake of [(14)C]-benzoic acid. Particularly, naringenin and silybin exhibited strong inhibition effects with IC50 values of 23.4 and 30.2 microM, respectively. Kinetic analysis indicated that the inhibition mode of naringenin and silybin on MCT1 activity was competitive with a Ki of 15-20 microM. The effect of flavonoids on the gene expression of MCT1 was also examined by using RT-PCR and western blot analysis. Results indicated that the expression level of MCT1 was not affected by the treatment with naringenin or silybin. The cellular accumulation of naringenin in Caco-2 cells was not changed in the presence of benzoic acid or L-lactic acid, implying that naringenin might not be a substrate of MCT1. In conclusion, some flavonoids appeared to be competitive inhibitors of MCT1, suggesting the potential for diet-drug interactions between flavonoids and MCT1 substrates.