A 16-week experiment was conducted on 224 three-day-old BUT-9 turkey chickens randomly assigned to 4 groups, each with 4 replicates; each group comprised 7 toms and 7 hens. In successive 4-week periods the basic ration was supplemented with 2 to 5% of a mixture of rape seed oil and poultry fat (66:34) differing in peroxide value in groups I-IV <5, 50, 100 and 150 mEq O₂/kg, respectively. The addition of oxidized fat had a negative effect on the growth rate of turkeys, causing significant (P<0.01) differences in the body weight of the birds at ages 12 and 16 weeks. At end of the experiment the birds in the control group (I) were an average 1.07 kg heavier than those in groups II-IV; these differences were considerably greater in tom turkeys and averaged 1.7 kg. The lower weight gain in the experimental birds was caused by the approximately 10% smaller consumption of feed than in the control group. Increasing the peroxide value of added fat from 50 mEq O₂/kg (group II) to 100 or 150 mEq O₂/kg (groups III and IV) did not further reduce body weight. The degree of fat oxidation did not distinctly affect mortality, feed consumption, or slaughter value of the turkeys. The addition of oxidized fat significantly lowered serum and hepatic vitamin E levels and, to a smaller extent, also the vitamin A content in the liver.