Metal hyperaccumulating plants should have extremely efficient defence mechanisms, enabling growth and development in a polluted environment. Brassica species are known to display hyperaccumulation capability. Brassica juncea (Indiana mustard) v. Malopolska plants were exposed to trace elements, i.e., cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn), at a concentration of 50 M and were then harvested after 96 hours for analysis. We observed a high index of tolerance (IT), higher than 90%, for all B. juncea plants treated with the four metals, and we showed that Cd, Cu, Pb and Zn accumulation was higher in the above-ground parts than in the roots. We estimated the metal effects on the generation of reactive oxygen species (ROS) and the levels of protein oxidation as well as on the activity and gene expression of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX). The obtained results indicate that organo-specific ROS generation was higher in plants exposed to essential metal elements (i.e., Cu and Zn), compared with non-essential ones (i.e., Cd and Pb), in conjunction with SOD, CAT and APX activity and expression at the level of encoding mRNAs and existing proteins. In addition to the potential usefulness of B. juncea in the phytoremediation process, the data provide important information concerning plant response to the presence of trace metals.