Baicalin is a major bioactive component of Scutellaria baicalensis and a substrate of multiple drug resistance-associated protein 2. Expression of multiple drug resistance-associated protein 2 is regulated by NF-E2-related factor 2. The aim of this study was to explore whether ursodeoxycholic acid, an NF-E2-related factor 2 activator, could influence the oral bioavailability of baicalin. A single dose of baicalin (200mg/kg) was given orally to rats pretreated with ursodeoxycholic acid (75mg/kg and 150mg/kg, per day, intragastrically) or normal saline (per day, intragastrically) for six consecutive days. The plasma concentration of baicalin was measured with the HPLC method. The result indicated that the oral bioavailability of baicalin was significantly and dose-dependently reduced in rats pretreated with ursodeoxycholic acid. Compared with control rats, the mean area under concentration-time curve of baicalin was reduced from 13.25 +/- 0.24mg/L h to 7.62 +/- 0.15mg/L h and 4.97 +/- 0.21mg/L h, and the C-max value was decreased from 1.31 +/- 0.03mg/L to 0.62 +/- 0.05mg/L and 0.36 +/- 0.04mg/L in rats pretreated with ursodeoxycholic acid at doses of 75mg/kg and 150mg/kg, respectively, for six consecutive days. Hence, ursodeoxycholic acid treatment reduced the oral bioavailability of baicalin in rats, probably due to the enhanced efflux of baicalin from the intestine and liver by multiple drug resistance-associated protein 2.