Plants of Bacopa rnonnieri were treated with six different concentrations of Hg (0.01, 0.5, 1 .O, 2.0, 3.0, and 5.0 p g mL-l) for 4, 7, and 14 d under laboratory conditions. The metal accumulation in the root tissues was about five times more than in the shoots. At all the concentrations of Hg, cysteine, total -SH, reduced glutathione, and ascorbic acid contents increased in the roots of 5. monnieri up to 7 d of exposure except the ascorbic acid content, which decreased above 2 p g mL-' Hg. The rnalondialdehyde content decreased at all the concentrations of Hg; however, protein and sugar contents decreased above 0.1 and 2.0 p g mL-' Hg, respectively, after 4 d of exposure in the roots of the plant. The chlorophyll content in leaves decreased significantly with an increase in metal concentrations and durations of exposure. The results suggest that an increase in cysteine, total -SH, reduced glutathione, and ascorbic acid content by Hg treatment at the initial exposure period are part of the overall expression of Hg tolerance in the plant, and the decrease in chlorophyll and protein content is a consequence of Hg toxicity at higher metal concentrations and increased period of exposure.