In this study, the effects of enrofloxacin, ciprofloxacin and norfloxacin at therapeutic doses and over therapeutic periods on plasma malondialdehyde (MDA) levels and erythrocyte catalase (CAT) activity were investigated. For this purpose, a hundred and sixty 3-day-old female Ross breed broiler chicks were used. Four groups, each comprising 40 were established. While group 1 was maintained as control, groups 2, 3 and 4 were administered enrofloxacin, ciprofloxacin and norfloxacin, respectively, at 10 mg/kg bw/day in drinking water. On days 1, 3, 5 and 7, ten animals were killed from each group, and plasma MDA levels and erythrocyte CAT activity were measured spectrophotometrically. The data obtained demonstrated that CAT activity was statistically decreased in groups 2 and 4 and increased in group 3 on day 1. In conclusion, it was determined that neither of the three drugs generated free radicals, in other words, caused lipid peroxidation physiologically when given at therapeutic dosage. (C) 2009 Elsevier Ltd. All rights reserved.