In this study, the effect of 24-Epibrassinolide (EBL) on antioxidant system in Arabidopsis thaliana were investigated under arsenate [As(V)] stress. The enzyme activity of superoxide dismutase (SOD) and catalase (CAT), total antioxidant status, malondialdehyde (MDA) level and free proline content, as well as the expression levels of SOD isoforms (Cu-ZnSODs, FeSODs and MnSOD), CAT isoforms (CAT1, CAT2 and CAT3), some heat shock proteins (Hsp70-4 and Hsp90-1) and proline biosynthesis (P5CS1 and P5CS2) genes were determined in rosette leaves of eight-week old plants under exposure of 100 and 200 mu M As(V) and/or 1 mu M EBL treatments for 24 h. Total SOD and CAT enzyme activities increased as a result of 100 mu M As(V) + EBL treatments compared to 100 As(V) treatment. Total antioxidant and proline levels increased in plants subjected to As(V), and the treatment of EBL together with stress caused further increase. As the MDA level increased in As-treated plants, 100 mu M As(V) + EBL treatment decreased MDA level. Transcript levels of CSD1, CSD2, FSD1, FSD2, MSD1 and CAT2 genes increased as a result of combined treatment of EBL and As(V) compared to control and alone stress treatments (except CSD1 gene). Expression level of CSD3, CAT1 and CAT3 genes were downregulated in response to As(V) and/or EBL treatments. EBL application alone and in combination with As(V) elevated the expression level of P5CS1 gene dramatically. Treatment with 100 mu M As(V) and EBL increased the transcript level of Hsp70-4 and Hsp90-1 genes in leaves compared to 100 mu M As(V) treatment. To our best knowledge, this is the first detailed study to evaluate the improving effect of EBL on antioxidant defense system at biochemical and transcriptional level in A. thaliana plants under As(V) stress.