Single intraperitoneal injections of sodium arsenite were given to albino Swiss-Webster mice on one of days 7–12 of gestation. Two dose levels were used: 10 or 12 mg/kg. Arsenite treatment resulted in high rates of fetal deaths and tended to decrease fetal weights-compared with H2O injected controls. Arsenite induced a variety of fetal malformations on gestation days 7–10; the most common were exencephaly, micrognathia, open eye, tail defects and skeletal anomalies of the ribs and vertebrae. Defects were similar to but less numerous than those caused by sodium arsenate.