Proteases are crucial for living cells and play a role in plant cell adaptation to environmental conditions. Oxidative stress produced oxidized proteins which are selectively degraded by proteases. To understand the role of proteolysis in response to metal stress, sunflower plants (a plant suitable for phytoremediation) were treated with 100 µM of CdCl₂, CuCl₂, AlCl₃, CoCl₂, PbCl₂, CrCl₃, NiCl₂, HgCl₂ or ZnCl₂. Changes in protease activity, gelatinase profile and protein oxidation were examined in sunflower cotyledons. Our results indicate that this tissue has mainly acid proteases belonging to different classes. Although all metals (except Zn) increased protein oxidation (62, 57, 112, 74, 74, 68, 64 and 40% for Pb, Al, Ni, Cd, Hg, Co, Cr and Cu over the control), they altered proteolysis in different ways. Pb, Al and Ni treatment decreased protease activity 22, 28 and 30% respect to control while Cd and Hg increased this activity in 23 and 27%. In Zn, Cu and Co treatments protease activity remained similar to control treatment. These results indicate that different proteases are involved in plant defence against metal toxicity. However, the identification of specific oxidized proteins involved in this process and the metal effect on class specific proteases should provide greater information.