AIM: To study the anti-hepatoma efficiency of arsenic trioxide (As2O3) in the treatment of experimental rat hepatocellular carcinoma (HCC) induced by 2-acetamidofluorene (2-FAA)and to elucidate the possible mechanisms.METHODS: SD rats (2 mo old) had been fed with 2-FAA for 8 wk to induce HCC, and then they were treated with As2O3 or matrine. On d 29, the rats were killed and the liver was weighed and liver tumors were counted. The histological changes of liver tissue were observed under microscope, and the cellular dynamic parameters were studied by flow cytometry. Immunohistochemistry (two-step method) was used to observe the expression of vascular endothelial growth factor (VEGF) and micro-vessel density (MVD) on consecutive sections. The pathological parameters were also analyzed, the levels of serum aspartate aminotransferase (AST), alanine aminotransferase (ALT),total bilirubin (TBi), and direct bilirubin (DBi).RESULTS: The number of liver tumors decreasedsignificantly in groups treated with As2O3, especially in medium-dose (1 mg/kg) group (t = 2.80, P＜0.01). As2O3 caused HCC cell death via apoptosis; necrosis was seen and apoptosis was common when the dose was 1 mg/kg.Proliferation index decreased sharply in medium-dose (1 mg/kg) group (7.87±4.11 vs 24.46±6.49, t = 2087,P＜0.01), but not in 0.2 mg/kg group. However, S-phase fraction decreased dramatically in both groups, it reached the bottom level only when the dose was 1 mg/kg compared with control (0.40±0.13 vs 3.01±0.51, t = 2.97, P＜0.01),and it was obviously accompanied with accumulation of cells in G0/G± (G0/G1 restriction). The expressions of VEGF and MVD in medium-dose (1 mg/kg) group were significantly lower than normal saline group (0.63±0.74 vs 2.44±0.88, P＜0.05; 15.75±3.99 vs47.44±13.41, t= 2.80,P＜0.01). Compared with normal saline group, mediumand low-dose groups As2O3 and matrine lowered the levels of ALT in serum (61.46±9.46, 63.75±20.40, 61.18±13.00 vs 108.98±29.86, t= 2.14, P＜0.05), but had no effect on the level of serum AST, TBi, and DBi.CONCLUSION: As2O3 had inhibitory effect on growth of experimental HCC in rats induced by 2-FAA, but had no obvious effect on normal hepatic cells. The mechanisms may involve decrease of cell division, accumulation of cells in G0/G1 phase, apoptosis of tumor cells, and inhibitory effect on angiogenesis through blocking VEGF.