Effect of boron toxicity on antioxidant response of sensitive (Hamidiye) and tolerant (Anadolu) barley (Holgarium vulgar) cultivars was investigated. Eight days old hydroponically grown seedlings were subjected to 5 and 10 mM boric acid treatments for 5 days. Growth parameters (dry-wet weight), protein, proline, MDA, H2O2 contents, membrane damage and activities of superoxide dismutase (SOD). ascorbate peroxidase (APX), catalase (CAT) and glutathion reductase (GR) was evaluated in root and shoot tissues. Compared with controls (no boric acid treatment) boron toxicity resulted in a reduction in root weights and did not cause any significant change in protein contents. Boric acid treatment did not cause significant (P > 0.05) changes in proline and H2O2 contents of both tissues and cultivars. On contrary, NIDA contents and electrolyte leakage exhibited a dose dependent increase in shoots whereas no significant change was observed in root tissues. Higher extend of membrane damage was observed in leaves of the sensitive cultivar. Total SOD, CAT and GR activities in shoot tissue of both cultivars did not change significantly whereas significantly higher APX activities was observed at 10 mM boric acid treatments. Significant increases in total SOD, CAT and decrease in GR activities was observed in roots of the sensitive cultivar. Roots of resistant cultivar, on contrary, exhibited enhanced CAT and decreased APX activity whereas no significant changes was observed in total SOD and GR activities. Our data suggest that. boron toxicity induced membrane damage in barley leaves, do not involve active oxygen species and antioxidant enzyme activity is not a critical factor in boron toxicity tolerance mechanism. To the best of our knowledge, this is the first report on antioxidant response of barley seedlings under boron toxicity. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.