Epidemiological studies have suggested synergistic interactions between chronic hepatitis B virus (HBV) infection and aflatoxin B 1 (AFB 1 ) exposure in the etiology of hepatocellular carcinoma (HCC), although the molecular mechanisms of their interactions are still not understood. The aim of this study was to use the Pekin duck model to investigate the impact of AFB 1 exposure on duck hepatitis B virus (DHBV) replication during the early stages of virus-carcinogen interactions. Six-week-old chronic DHBVcarrier or uninfected ducks were exposed to AFB 1 for 5 weeks or treated with dimethylsulfoxide (DMSO) as a control. Animals were observed for 6 to 13 weeks after AFB 1 treatment to study the influence of AFB 1 exposure on DHBV replication and liver pathologies. Histological analysis showed more marked changes in the livers of AFB 1 -treated ducks, and these were enhanced by DHBV infection. A significant increase in serum and liver DHBV DNA level was observed in AFB 1 -treated ducks as compared with DMSO-treated controls. In addition, viral RNAs, in particular the pregenomic RNA that is the template of viral replication, and intrahepatic DHBV DNA replicative intermediates, were significantly increased by AFB 1 treatment. Moreover, an overexpression and accumulation of DHBV large envelope (L) protein was observed in the hepatocytes of AFB 1 -exposed animals. The in vitro study has further confirmed an increase in intracellular viral DNA and in virus release in AFB 1 -treated primary duck hepatocytes. Taken Chronic hepatitis B virus infection (HBV) and exposure to aflatoxin B 1 (AFB 1 ) are two major risk factors that have been associated with the etiology of hepatocellular carcinoma (HCC) in humans in high-incidence areas (Africa and Southeastern Asia). 1 Moreover, several epidemiological studies in China 2,3 and in Taiwan 4 have shown that these two factors can have synergistic effects on the risk of HCC.Animal models have a pivotal role to play in understanding interactions between AFB 1 and chronic HBV infection. An earlier onset of liver tumors was observed in transgenic mice overexpressing HBV large envelope (L) protein and treated with AFB 1 as compared with the same mice not exposed to this carcinogen. 5,6 In the woodchuck model, administration of AFB 1 to woodchuck HBV-carrier animals led to a significantly earlier appearance of hepatocellular neoplasms and a higher incidence of HCC, indicating the synergistic effect of viral infection and carcinogen exposure in hepatocarcinogenesis. 7 The Pekin duck is also of interest for the study of such interactions, because it is highly sensitive to the carcinogenic effects of AFB 1 and can be infected with duck HBV (DHBV). Naturally arising liver cancers have been reported in domestic ducks in Qidong, China, where the human HCC rate is particularly high, and where exposure of humans and ducks to AFB 1 has been demonstrated. [8][9][10][11][12] Experimental administration of AFB 1 to ducks led to development of HCC with no significant differences between DHBV-infecte...