Dominant-lethal effects of 10 mg/kg methylmercuric hydroxide were studied in male mice from two hybrid stocks and in females from one of these stocks. Two other compounds, mercuric chloride (2 mg/kg) and cadmium chloride (2 mg/kh), were studied only in females for dominant-lethal (in one hybrid stock) and reproductive capacity effects (in two hybrid and one mixed stocks). All compounds were administered in a single intraperitoneal injection. When males of one of the two stocks studied were treated with methylmercuric hydroxide, the females to which they were mated exhibited a slight reduction in the total number of implantations and in the number of living embryos. These reductions were accompanied by a very small increase in the incidence of dead implantations. In females, cadmium chloride had no detectable dominant-lethal or other fertility effects, except superovulation. On the other hand, the two mercury compounds slightly reduced the numbers of implants and living embryos in females subjected to dominant-lethal studies. The two mercury compounds also induced a slight reduction in that long-term reproductive performance of one stock of females. These results and those reported earlier by others, indicate that the mercury compounds studied so far are not potent inducers of dominant-lethal mutations in male and female mice. It is not clear whether the small effects on male or female fertility induced in some cases, particularly the increase in dead implantations and reductions in the number of living embryos, were attributable to dominant-lethal mutations or to nongenetic causes.