The traditional Chinese medicine Scutellaria radix has been used for thousands of years, mainly for the treatment of inflammatory conditions including hepatitis. The major active constituent, wogonin (WG), isolated from S. radix has attracted increasing scientific attention in recent years due to its potent biological activities. However, pharmacologic studies have primarily been focused on wogonin's anti-inflammatory and anti-cancer activities. In this study, we have investigated wogonin's anti-hepatitis B virus (HBV) activity both in vitro and in vivo. In the human HBV-transfected liver cell line HepG2.2.15, wogonin effectively suppressed the secretion of the HBV antigens with an IC(50) of 4 microg/ml at day 9 for both HBsAg and HBeAg. Consistent with the HBV antigen reduction, wogonin also reduced HBV DNA level in a dose-dependent manner. Duck hepatitis B virus (DHBV) DNA polymerase was dramatically inhibited by wogonin with an IC(50) of 0.57 microg/ml. In DHBV-infected ducks wogonin dosed i.v. once a day for 10 days reduced plasma DHBV DNA level with an ED(50) of 5mg/kg. The in vivo anti-HBV effect of wogonin in ducks was confirmed by Southern blotting of DHBV DNA in the liver. Histopathological evaluation of the liver revealed significant improvement by wogonin. In addition, in human HBV-transgenic mice, wogonin dosed i.v. once a day for 10 days significantly reduced plasma HBsAg level. Immunohistological staining of the liver confirmed the HBsAg reduction by wogonin. In conclusion, our results demonstrate that wogonin possesses potent anti-HBV activity both in vitro and in vivo. Currently, wogonin is under early development as an anti-HBV drug candidate.