This study was carried out to investigate the neurotoxic and immunotoxic effects of Indole-3-butyric acid (IBA), a plant growth regulator (PGR), on rats at subacute and subchronic exposure. The neurotoxic effects of IBA were evaluated by measuring the activities of acetylcholinesterase (ACNE) and butyrylcholinesterase (BChE). Biomarkers selected for immunotoxic monitoring were the activities of adenosine deaminase (ADA) and myeloperoxidase (MPO) 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 decreased ACNE and BChE activities in some tissues of the rats treated with both dosages and periods of IBA. With regard to the immunotoxic effects, ADA activity significantly decreased whereas MPO activity increased after subacute and subchronic exposure with both dosages in most of the tissues of rats compared with controls. The observations presented led us to conclude that the administrations of IBA at subacute and subchronic exposure decreased ACNE, BChE and ADA activities whereas increased MPO activity in various tissues of rats. This may reflect the potential role of these parameters as useful biomarkers for toxicity of IBA. (C) 2009 Elsevier Inc. All rights reserved.