Background/aims: We aimed to determine the progress of lipid peroxidation and ultrastructural changes established in the rat liver after acute bile duct ligation. Methods: Groups A1, B1, C1 and D1 were the controls of groups A2, B2, C2 and D2, which represented the 1st, 3rd, 5th and 8th days after bile duct ligation. Serum bilirubin and malondialdehyde, liver malondialdehyde and reduced glutathione levels, and inducible nitric oxide synthase expression were determined. Liver tissue was examined with light and electron microscopy. Results: Serum bilirubin increased progressively. Serum and liver malondialdehyde and inducible nitric oxide synthase expression reached a peak level at day 3, reduced at the 5th day and continued at a constant rate. Reduced glutathione decreased progressively. Ductal proliferation increased progressively to a plateau at day 5. The marked electron microscopic changes were detected at day 3 (B2) and continued constantly. Conclusions: The first five days after acute bile duct ligation are the most critical.