To evaluate the effects of duck hepatitis virus-1 (DHV-1) on the body weight gain in duck and the effects of silymarin on it in vivo, 100 10-d-old ducks, both male and female, were collected to be subjected to the test. The experiments were conducted in 8 groups: in group 1-3, the animals were inoculated with 1:105 diluted duck hepatitis virus (DHV-1) infected allantoic fluid and given 0, 30, and 50 mg kg-1 BW d-1 silymarin orally, respectively. In group 4-6, the animals were inoculated with 1:5×105 diluted DHV-1 infected allantoie fluid and given 0, 10, and 30 mg kg-1 BW d-1 silymarin orally, respectively. In group 7, the animals were given 10 mg kg-1 BW d-1 silymarin only. Group 8 was the control one treated by injecting sterillized saline into the leg muscles. All the silymarin was given from 0 to 4 d after inoculation of the virus. By the 5th d after inoculation, the vein blood was drawn from the dorsal foot vein and the plasma samples were collected and stored at -20℃. The body weight gain (BWG) was measured from 0 to 10 d after inoculation. The plasma IGF-I, T3, and T4 concentrations were measured by radioimmunoassay (RIA). At the virus dose of 1:5×105 diluted virus infected allantoic fluid, the inoculation of the virus enhanced the BWG significantly compared with that of the control (P< 0.01), while 10-50 mg kg-1 BW d-1 silymarin could counteract the effects of the virus on the BWG dose-dependently. The plasma IGF-I levels showed no correlation with the BWG, but the T3 levels showed a same tropism with the body weight gain. The present results indicated that sublethal DHV-1 enhanced the body weight gain of ducklings significantly, and the silymarin could counteract this effect in vivo.