BACKGROUND: Hepatic stimulator substance (HSS) is a known hepatic growth factor that appears to be organ-specific but species-nonspecific. In the present study we investigated the effect of HSS administration in a rat model of liver injury and regeneration induced by thioacetamide (TAA) injection. METHODS: TAA (300 mg/kg body weight) was injected intraperitoneally in male Wistar rats (group I). HSS (50 mg protein/kg body weight) was administered intraperitoneally either at 24 h (group II) or at 36 h (group III) after TAA treatment. The animals were killed at different time points after TAA injection, and the rate of tritiated thymidine incorporation into hepatic DNA, the activity of the enzyme thymidine kinase in liver, and the assessment of the mitotic index in hepatocytes were used to estimate liver regeneration. RESULTS: The administration of TAA caused severe hepatic injury recognized histopathologically and by increased activities of the serum hepatic enzymes aspartate and alanine aminotransferases. The hepatic injury, which peaked at 24 h and 36 h after TAA injection, was followed by a regenerative process of hepatocytes which presented peaks after 48 h and 60 h (group I). The regenerative process of hepatocytes remained unaffected when HSS was administered 24 h after the injection of TAA (group II). In the case of HSS administration 36 h after the injection of TAA (group III) the examined indices of hepatocyte proliferation were statistically significantly increased at 48 h (P < 0.001), compared with those observed in group I. CONCLUSIONS: The administration of HSS enhanced the hepatocyte proliferative capacity, induced by TAA treatment, depending on the time of its administration.