Ninety six 3-day-old Bowans White strain chicks were divided into a control and 7 experimental groups. Group 1 (control) received normal drinking water for 7 d. Chicks from groups 2, 3 and 4 received 50 ppm of toltrazuril at the begining and on days 3 and 6 of the experiment. Groups 5, 6, 7 and 8 were infected with a single dose of Eimeria tenella oocysts and then groups 6, 7 and 8 received additionally 50 ppm of toltrazuril on days 0, 3 and 6 after the infection.. At the end of the 7(th) d, all the animals were sacrified, their blood were collected into heparinised tubes and the plasma malondialdehyde (MDA) level and the erythrocyte catalase (CAT) activity were examined. The obtained results revealed that E. tenella oocysts caused lipid peroxidation in chicks and the administration of toltrazuril relieved the oxidative damage. Therefore, in case of infection, the detected level of plasma MDA and the activity of erythrocyte catalase may be taken into consideration as references with other parameters, in order to determine the severity of infection and the effectiveness of treatment.