The effect of methylmercury (MeHg) on reactive oxygen species (ROS) induction in neural cell lines was measured by the fluorescent probe, chloro methyl derivative of di-chloro di-hydro fluoresceindiacetate (CMH2DCFDA). Three different MeHg concentrations (5, 10 and 25 microM) and time periods (30, 50 and 90 min) were studied in C6-glial and B35-neuronal cell lines. In addition, the relationship between MeHg-induced ROS and cell density (day 3 vs. day 4) was also explored. The 14C-labelled MeHg measurements were done to determine the cell associated-MeHg content. At 30 and 50 min exposure, a significant increase (p<0.05) in MeHg-induced ROS was observed at 10 and 25 microM MeHg for C6 cells and at 25 microM MeHg for B35 cells. However, the amount of ROS produced with 25 microM MeHg varied significantly (p<0.001) at different time periods. For both the cell lines, significant cell density dependent differences (p<0.05) were observed at 10 microM MeHg treatment for 50 min. MeHg treatments were associated with a concentration as well as cell-density dependent increase in cell associated-MeHg. These findings provide experimental evidence that special attention should be focused upon concentration, exposure time and cell density when assessing MeHg-induced ROS via fluorescence.