Increasing evidence suggests that oxidative stress may play a significant role in causing microcystin (MCs) toxicity not only in mammals, but also in fish. MCs are a family of cyclic peptide toxins produced by some species of freshwater cyanobacteria (blue-green algae). Among the microcystins, MC-LR is the most extensively studied. In the present study the differential response of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione reductase (GR) as well as lipid peroxidation (LPO) as a biomarker of oxygen-mediated toxicity were assessed in liver, kidney and gill tissues of tilapia (Oreochromis sp.) exposed to MCs. Fish were injected intraperitoneally (i.p.) with a single dose of 500 microg/kg MC-LR or 500 microg/kg MC-RR and sacrificed after 7 days. The results show that MCs exposure induces adaptive responses such as increase in the antioxidant enzymatic activities, mainly those of SOD and CAT, as well as in LPO values. With regard to LPO values, the liver was the most affected organ by MC-LR. MC-RR, however, did not affect this parameter in the liver of the exposed fish. Oxidative stress biomarkers, therefore, are valuable tools in the assessment of early responses of fish to the increasing occurrence of cyanobacterial blooms worldwide.