Domoic acid (DA) is a neurotoxicant produced by Pseudo-nitzschia from diatomeae. Although the neurotoxic and genotoxic effects of DA have been well documented, the number of in vivo studies regarding the oxidative stress inducing effects of DA is quite limited. In this study, in vivo toxic effects of DA were investigated on fish Oreochromis niloticus (Fam: Cichlidae), using oxidative stress biomarkers. Fish were exposed to three different concentrations (1, 5 and 10 mu g/g body weight) of DA via intraperitoneal injections and the tissues were sampled at 24, 48 and 72 h post-treatment. Changes in the level of lipid peroxidation, and activities of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glutathione reductase (GRd) were evaluated in liver and gill tissues. Our results revealed dose and time dependent increases in the oxidative stress parameters. It was also observed that the toxic effects were more pronounced in liver than in gill tissue. (C) 2009 Elsevier Ltd. All rights reserved.