Mercury (Hg) is one of the most important environmental pollutants that negatively affects plant growth and development. Cysteine (Cys) plays an important role in plant response to various environmental stress factors. In the present study, the alleviation of Hg stress through exogenous Cys treatment to maize seedlings was evaluated. For this purpose, a hydroponic experiment was set up to investigate the effect of HgCl2 (100 mu M) and in combination with Cys (200 mu M) on plant growth, total chlorophyll content, reactive oxygen species, antioxidant enzyme activities, and mRNA expression levels of some antioxidant genes in maize seedlings. The results showed that HgCl2 treatment significantly decreased both root and shoot growth and total chlorophyll content, also increased the malondialdehyde (MDA), hydrogen peroxide (H2O2), and superoxide levels (O-2(center dot)) in maize seedlings. After treatment with 200 pM exogenous Cys combined with 100 mu M HgCl2, root and shoot growth and total chlorophyll content increased and the concentration of MDA, H2O2, O-2(center dot-) in seedlings notably decreased and catalase (CAT), glutathione reductase (GR), superoxide dismutase (SOD), and peroxidase (POD) activities in seedlings increased significantly. In addition, HgCl2 treatment alone or combination with Cys markedly increased the root and leaf Hg content. 'I he higher amounts of Hg accumulated in the roots of the seedlings. Furthermore, qRT-PCR results showed that the m RNA levels of CAT, GR, and SOD genes were up-regulated at HgCl2 + Cys treatment groups compared to the HgCl2 treatment alone. The results of the study indicated that exogenous Cys improved resistance to Hg-stress in maize seedlings by activating antioxidant defence system.