Aim: The aim of our study was to investigate hepatoprotective properties of bile acids (BA), natural ursodeoxycholic (UDK) and semisynthetic 12-monoketocholic acid (MK) by testing the levels of liver enzymes, malonyldehyde (MDA) as an indicator of hepatocyte oxidative damage, activity of antioxidant enzymes, histological analysis of liver tissue and immunohistochemical analysis of hepatocyte proliferation, activation of nuclear farnesoid-X receptor (FXR), apoptosis and proliferation markers. Introduction: The hepatocyte metabolism is extremely intense, and the processes that take place in hepatocytes are diverse, which makes them highly susceptible to oxidative damage that is the basis of many diseases. Two popular models of oxidative stress induction are the use of ethinylestradiol and the cholestasis model, as well as the use of aloxan and aloxan-diabetes model. Results: Male Wistar rats were used in this study. Control group (K) received only saline (0.5 ml/kg). Three groups received ethinylestradiol (EE group, 0.5 mg/kg) as monotherapy or in combination with 12-monoketocholic (E-MK, 4 mg/kg) or ursodeoxycholic acid (EPLENARY ORAL PRESENTATION 24 MATERIA MEDICA • Vol. 34 • Issue 1, suplement 1 • april 2018. UDK, 25 mg/kg). In order to increase the level of oxidative stress, in three groups monodose of aloxan was administered in addition to ethinyl estradiol, either alone (AE group), or treated additionally with 12-monoketocholic (AE-MK) and ursodeoxocholic (AE-UDK) acid at the above dosages. Liver tissue was sampled for histological analysis and biochemical analysis of liver enzymes and bilirubin (AST, ALT, GGT, bilirubin total (BLRu) and direct (BLRd)), oxidative damage (MDA), levels of antioxidant enzymes catalase (CAT), glutathione-peroxidase (GSH-Px), glutathione-reductase (GSH-R), glutathione-S-transferase (GSH-ST). In groups EE and AE, AST, ALT and GGT levels were significantly elevated compared to K group, while the use of bile acids led to a significant reduction in activity of all enzymes as well as total and direct bilirubin. Both acids have the strongest hepatoprotective effect in terms of AST activity and lowering the total bilirubin concentration, whose values using both UDC and MK were lowered to the level of K-group. In terms of ALT and GGT, bile acids showed slight selectivity. Histological analysis of liver in groups EE and AE showed hepatocytes with karyopycnosis and cytoplasmic acidophilia, which indicate the apoptotic cell death, as well as changes in individual hepatocytes corresponding to feathery degeneration. All of these changes weren`t observed in the liver of animals treated with UDK and MK. By analyzing Ki-67 in the tissue of the control group, it was found that up to 5% of the cells exhibited nuclear expression, and a value of 10% was taken as the limit for the enhanced proliferative activity of hepatocytes. In groups that received only hepatotoxic substances, ethinyl estradiol and aloxan, the expression of ki-67 was low as in controls. Animals whose hepatocyte damage was caused only by ethinylestradiol, reflected hepatoprotective properties of bile acids by enhancing hepatocyte proliferation (E-UDK in 50%, and E-MK in 100% of animal). Similar hepatoprotective effects of tested bile acids was seen when comparing group treated with both hepatotoxic substances (AE), and groups cotreated with UDK (all animals had an increased proliferative activity) and MK (80% of animals). Accumulation of hydrophobic acids in ethinylestadiol-induced cholestasis enhances oxidative damage and induces apoptosis, but also activates the expression of nuclear farnesoid-X-receptor. In the K-group, 50% of the animals showed low expression of FXR, and moderate and high expression was present in 25% of animals. In EE and AE groups, high FXR expression was significantly increased (75%, ie 60%) indicating the accumulation of hepatotoxic acids that raise the level of oxidative damage and induce apoptosis. The hepatoprotective effect of UDK and MK, has been demonstrated by the reduced content of hydrophobic acids, ie, the reduced expression of FXR. In groups in which hepatotoxicity and oxidative stress were induced only by ethinylstradiol and were treated with UDK and MK, not a single animal had high FXR expression, and a number with weak expression increased significantly. Similarly, in groups that received two toxic substances and analyzed acids (AE-MK, AE-UDK) 60% of animals had weak FXR expression, which is statistically not different from the control group. Groups treated with ethinylestadiol and aloxan had a significantly higher concentration of malonylaldehyde (MDA), an indicator of oxidative hepatocyte damage. The use of UDK and MK significantly reduced the concentration of MDA, but not to the control group level. The activity of antioxidant enzymes in the EE group did not differ statistically from control, and the use of UDK and MK resulted in significant changes. In the group AE where oxidative stress is enhanced, the activity of all analyzed enzymes is significantly decreased, and the application of UDK and MK significantly enhances the activity of the antioxidative enzymes. In order to examine the hepatotoxicity of oxidative stress and hepatoprotective properties of UDK and MK, the expression of the p53 and Bcl-2 family of proteins was studied. Hepatocytes of the control group did not express the proapoptotic Bax, while in the EE and AE groups, weak expression was present in 50% and 40%, respectively. Using UDK in both EE-UDK and AE-UDK models, Bax was completely reduced to the level of control group, ie no Bax expression was observed. The use of MK in both models led to suppression of Bax (EE-MK, 25%; AE-MK, 0%). The antiapoptotic Bcl-2 protein was detected in the control group in 25% of the individuals, unlike the EE and AE groups, where the expression was completely suppressed. By promoting the expression of Bcl-2 (E-UDK in 25% of individuals, AE-UDK in 40% of individuals), the UDK showed a stronger hepatoprotective activity compared to MK which in the AE-MK group only led to an increase in expression by 20%. In the control group p53, expression was recorded only in the cytoplasm, in 50% of animals, while in the EE and AE groups p53 had nuclear expression in 25%, ie 20% of the individuals, which speaks in favor of more intensive apoptotic action and induction of transcription of other proapoptotic proteins. In groups where UDK was administered, p53 was not present in either the nucleus or the cytoplasm, while MK reduced the expression to the control group level. Conclusion: Ursodeoxycholic and 12-monoketocholic acid have great antioxidative and antiapoptotic capacity, which explains their hepatoprotective effect.