Selected biochemical indicators in blood and liver and indicators of femur mineralization were determined in rats fed for eight weeks on diets containing 10% fat with a peroxide value below 5, 40, 80, 120, 160, or 200 meq O₂/kg. The average body weight of the rats was 259.9±10.5 g, the experimental groups contained 12 animals. Fat with a high peroxide value (160 and 200 meq O₂/kg) significantly increased the concentration of malondialdehyde in the serum and slightly in the liver. The most sensitive indicators of the reaction of rats to oxidation of dietary fat were the activity of glutathione peroxidase in erythrocytes, which increased with the rising degree of dietary fat oxidation, i.e. at 40 meq O₂/kg, the activity of serum aspartate aminotransferase and the vitamin A content in the liver, which fell at a peroxide value of 80 meq O₂/kg. A less sensitive indicator was erythrocyte peroxide dismutase activity, which did not increase until fat with a peroxide value of 160 meq O₂/kg was fed. The degree of fat oxidation did not significantly affect the activity of serum alanine aminotransferase, liver enzymes (aspartate aminotransferase, alanine aminotransferase, acid phosphatase, and lactate dehydrogenase), serum and liver triglycerides levels, total cholesterol and HDL cholesterol, or femur mineralization indicators.