The effects of chronic treatments with HgCl2on cell survival, DNA replication, and cell progression in cultured Chinese hamster ovary cells were investigated. The ability of these cells to recover from the effects was also characterized. Exposure of cells to 4 × 10−5M HgCl2for 30 min killed about 50% of the cells, and this proportion did not increase with continued exposure up to 24 h. The rate of DNA replication was reduced to 28% of the control rate in the presence of HgCl2. However, when the cells were returned to medium without HgCl2, the rate of DNA replication recovered to 88% of control after 3 h of exposure and 55% of control after 8 h of exposure. The cell doubling time was increased from a control value of 16 h to 31 h in the presence of HgCl2. When the exposed cells were returned to medium without HgCl2, the doubling time returned to 16 h. The rate of progression of cells from G1phase to S phase was greatly reduced in the presence of HgCl2, and no recovery was observed in this case when the cells were transferred to normal medium. These findings suggested a correlation between ability to recover and the cytotoxic effects of HgCl2.