Methylmercury (MeHg) is well known as a neurotoxic chemical. However, little is mentioned about its neurotoxic mechanism or molecular target in human neuronal cells in particular. We show in this study that exposure of human neuronal cell line, SH-SY5Y, to MeHg dose- and time-dependently impairs viability and mRNA expression of selenoprotein W (SeW) with a significant difference, unlike other selenoenzymes such as, SeP, GPX4, 5DI, and 5'DI. Using real-time RT PCR, the influence of selenium (Se) and glutathione (GSH) on SeW expression was also investigated. While Se depletion caused a weakly reduced SeW mRNA levels, additional Se caused an increase of SeW mRNA levels. Although 2 mM GSH had induced a weak shift on SeW level, the expression of SeW mRNA was down-regulated in SH-SY5Y cells treated with 25 microM BSO, an inhibitor of GSH synthesis. To understand the relationship between a decrease of SeW expression and intracellular GSH and ROS, we measured the concentration of intracellular GSH and ROS in cells treated to 1.4 microM MeHg using fluorescence based assays. A positive correlation was found between SeW mRNA level and intracellular GSH but no significant correlation was observed between intracellular ROS and SeW mRNA level or intracellular GSH contents. Therefore, we suggest that SeW is the novel molecular target of MeHg in human neuronal cells and down-regulation of this selenoenzyme by MeHg is dependent not on generation of ROS but on depletion of GSH.