Inorganic arsenic, an environmental contaminant, produces a variety of stress responses in mammalian cells, including metabolic abnormalities accompanied by growth inhibition and carcinogenesis. Much of the toxicity of arsenic is known to stem from its uncoupling effects on mitochondria. Because previously we had shown that mitochondrial dysfunction can disrupt oocyte and embryo development, we investigated effects of arsenite on meiotic progression and early embryo development in mice. Six-week-old CD-1 mice were treated with 0 (solvent as control), 8 mg/kg (a dose previously established in mice as the maternal no-observed-adverse-effect level), and 16 mg/kg doses of sodium arsenite every 2 days for a total of seven i.p. injections ver a period of 14 days. The incidence of meiotic anomalies, characterized by spindle disruption and/or chromosomal misalignment, was significantly increased in arsenite-treated groups (25% after 8 mg/kg and 62.5% after 16 mg/kg), compared to normal metaphase II in control oocytes. Further, arsenite treatment significantly decreased cleavage rates of zygotes at 24 h, morula formation at 72 h, and development to blastocysts at 96 h in a dose-dependent manner. The total cell number in developed blastocysts did not differ significantly between the 8 mg/kg arsenite treatment and control groups, but was significantly reduced in the 16 mg/kg arsenite treatment group. Moreover, the percentage of apoptotic nuclei was significantly increased in blastocysts following 16 mg/kg arsenite treatment. These data suggest that arsenite causes meiotic aberrations, which may contribute to decreased cleavage and preimplantation development, as well as increased apoptosis.