This report compares the effects of methylmercuric chloride (MMC) and mercuric chloride (MC) on the development of mouse preimplantation embryos in vivo. Female mice were injected with a single intravenous dose of 0.5-20.0 mg Hg/kg MMC or 0.5-2.5 mg Hg/kg MC on day 0 of gestation. The embryos were recovered by flushing excised oviduct and uterus on day 3.5 of pregnancy, and were examined for abnormalities. In the groups treated with doses of 0.5 and 1.0 mg Hg/kg of both compounds, the rates of abnormal embryos were not significantly different from that in the control group. The 50% effective dose of MMC was twice as great as that of MC. With increasing dose, the difference became more obvious; the 80% effective doses differed by a factor of ten. The body weight of dams decreased in terms of the dose of mercury in MC-treated groups, but did not vary in MMC-treated groups. The sensitive developmental stage for mercury toxicities could not be determined clearly, although the high sensitivity was reported in the blastocyst stage in vitro. The embryos treated in vivo were less sensitive than those reported in vitro.