Objective To explore the influence of exposure to mercuric chloride by drinking water on percentage of T cells and B cells in spleen of mice. Methods Seven-week-old female BALB/c mice were randomly divided into control group and three mercuric chloride-treated groups at different time to body weight， 10 in each group. The control group drank double distilled water， and mercuric chloride-treated groups drank mercuric chloride solution of 15 mg/L. At 1， 2 and 3 weeks after exposure， the cytometry was used to measure the number of splenocytes， the percentages of T cells， B cells and T cell subpopulations in the spleen， and the percentage ofregulatory Tcells（Treg） among CD4⁺T cells. Results Mercury chloride significantly increased the spleen weight at the first week of exposure， and significantly increased the kidney weight at the third week of exposure. The number of splenocytes was not affected by mercuric chloride. The percentages of T cells[（27.98±3.06）%]， CD4⁺ T cells[（19.58±2.06）%] and CD8⁺ T cells[（9.62±1.10）%] in the spleen decreased significantly since the first week of exposure（all P<0.05）， and the changes were most obvious at the second weeks， on the time-dependent manner. Meanwhile， the effect of mercuric chloride on the percentage of B cells was opposite to T cells. At the first week of exposure， mercury chloride significantly increased the percentage of Treg [（14.25±1.41）%] among CD4⁺ cells. Conclusion Exposure to mercuric chloride via drinking water alters the percentages of T cells， B cells and T cell subpopulations in the spleen of mice， and increases the percentage of Treg among CD4⁺ cells.