1. In the present study, we investigated the immunosuppressive effects and mechanisms of action of emodin on acute graft rejection following liver transplantation in a rat model of orthotopic liver transplantation. 2. Rats were divided into three groups: Group A, syngenic control (Brown Norway-to-Brown Norway); Group B, acute rejection group (Lewis-to-Brown Norway); and Group C, emodin-treated group (Lewis-to-Brown Norway treated with 50 mg/kg emodin, 50 mg/kgxd, injected intraperitoneally once a day from days 1 to 5 posttransplantation). The survival time of the recipients in each group was recorded. Histopathological changes in the liver, hepatocellular apoptosis, serum concentrations of interleukin (IL)-2, interferon (IFN)-gamma and IL-4 and their expression in liver tissue were determined. 3. Emodin treatment prolonged liver allograft survival time from 10.9 days in Group B to 25.6 days in Group C. The rejection activity index (calculated according to the Banff Schema) in Groups A, B and C was 1.29 +/- 0.47, 7.58 +/- 0.85 and 4.72 +/- 0.79, respectively (P < 0.01 for Groups A and B vs Group C), whereas the apoptosis index in the three groups was 15.51 +/- 1.47, 39.50 +/- 1.65 and 16.72 +/- 1.73, respectively (P < 0.01 for Groups A and C vs Group B). Serum levels of IL-2 and IFN-gamma were higher, whereas levels of IL-4 were lower, in the acute rejection group (Group B) than in the emodin-treated group (Group C; P < 0.05). Changes in the expression of these cytokines in transplanted liver tissue were consistent with changes in serum concentrations. 4. In conclusion, emodin effectively suppresses acute graft rejection in vivo to prolong the survival of recipient rats. The mechanism underlying this effect may be associated with the prevention of hepatocyte apoptosis and with a changing in the balance of Th1/Th2 cytokines towards Th2.