This study was carried out to investigate the effects of Indole-3-butyric acid (IBA), a plant growth regulator (PGR), on antioxidant defense systems (ADS) such as reduced glutathione (GSH) level and Glutathione-S-transferase (GST), glutathione peroxidase (GSH-Px) superoxide dismutase (SOD) enzymes activity in various tissues of rats exposed to 25 and 50 ppm dosages of IBA for 20 and 45 days. Results showed that the administrations of IBA fluctuated GSH levels in some tissues of rats treated with both dosages and periods. With regard to the ADS enzymes, SOD and GST activities increased significantly in the most of the tissues in rats treated with both dosages and periods of IBA. Also, GSH-Px activity fluctuated after subacute and subchronic exposure with both dosages in some of the tissues in rats compared to that the control rats. The observations presented led us to conclude that the administrations of IBA at subacute and subchronic affected the ADS system in various tissues of rats. This may reflect the potential role of these parameters as useful biomarkers for toxicity of IBA. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.