In this study, it is aimed to investigate the effects of different amounts of oak leaves (Quercus hartwissiana) consumed with or without polyethylene glycol (PEG) on the levels of reduced glutathione (GSH), malondialdehyde (MDA), beta-carotene and retinol. Fourty two lambs with the average body weight 33.0 +/- 0.7 kg were used as the animal material in the study. The animals were divided into 7 groups each containing 6 lambs. The lambs housed in individual cages were fed with 272 g concentrate feed and various amounts of dried hay with the following feeding regimen: Group 1: Standard ration (Control); Group 2: 185 g oak leaf plus 0 g PEG; Group 3: 185 g oak leaf plus 10 g PEG; Group 4: 185 g oak leaf plus 20 g PEG; Group 5: 370 g oak leaf; Group 6: 370 g oak leaf plus 20 g PEG; Group 7: 370 g oak leaf plus 40 g PEG. Blood samples were collected from all animals on day 0 (control), 30 and 60. Levels of whole blood GSH, plasma MDA, beta-carotene and retinol were measured colorimetrically. It was determined that oak leaves added to rations as roughage did not affect GSH, MDA, retinol and beta-carotene levels in the presence or absence of PEG. In daily comparison of groups, only group 4 (60th day) had significantly decreased MDA levels as compared to others (P<0.05). According to the results, it could be concluded that oak leaves do not cause alterations in the selected antioxidant parameters in lambs.