Exogenous brassinosteroid (BR) has been reported to improve plant resistance to abiotic stress, but little is known about the role of endogenous BR in plant stress responses. In this study we investigated the involvement of endogenous BR in salt stress response using BR mutants det2-1 and bin2-1 of Arabidopsis. Seed germination and seedling growth of det2-1 and bin2-1 were more sensitive to salt stress than that of Columbia wild type (WT). The transcript levels of salt- and ABA-induced genes COR78 and P5CS1 were less induced in det2-1 than in WT under 200 mM NaCl. In addition, the basal proline level and, to a lesser extent, the proline level induced by 200 mM NaCl or 50 mu M ABA in both det2-1 and bin2-1 was enhanced, resulting in decreased proline accumulation. On the other hand, exogenous 24-epibrassinolide (EBR) could enhance proline accumulation, promote root elongation of WT, and partially rescue the growth of det2-1 under salt stress. These results suggested that endogenous BR is positively involved in the plant response to salt stress in Arabidopsis.