Entecavir (ETV), a cyclopentyl guanosine nucleoside analog, was evaluated in transgenic mice expressing hepatitis B virus (HBV). ETV administered orally once daily for 10 days at a dosage of 3.2mg/kg significantly (P<or=0.001) reduced liver HBV DNA in female mice from 5.9 to <0.82 pg of HBV DNA per microg of cellular DNA, and from 8.3 to <1.1 pg/microg in male mice. To compare the efficacy of ETV with other compounds previously evaluated in this model and with ETV activities in other animal models, the efficacy of serial one-half log dilutions of ETV were evaluated in both male and female mice to determine the minimal effective dose. End-point titration experiments resulted in a statistically significant HBV DNA reduction in the liver at concentrations of 0.032 and 0.1mg/kg per day in female and male mice, respectively. Viral liver RNA, and serum e (HBeAg), serum surface (HBsAg), and liver core antigens (HBcAg) were not affected by ETV treatment presumably because the antiviral target was viral polymerase activity and the HBV produced from the transgene was not capable of secondary rounds of infection in the mouse. ETV was well tolerated and no morbidity or mortality was observed during the 10-day study. Similar to other animal models, ETV displayed potent anti-HBV activity in this transgenic mouse model.