Information on the reproductive and developmental toxicity of inorganic arsenic is available primarily from studies in animals using arsenite and arsenate salts and arsenic trioxide. Inorganic arsenic has been extensively studied as a teratogen in animals. Data from animal studies demonstrate that arsenic can produce developmental toxicity, including malformation, death, and growth retardation, in four species (hamsters, mice, rats, rabbits). A characteristic pattern of malformations is produced, and the developmental toxicity effects are dependent on dose, route, and the day of gestation when exposure occurs. Studies with gavage and diet administration indicate that death and growth retardation are produced by oral arsenic exposure. Arsenic is readily transferred to the fetus and produces developmental toxicity in embryo culture. Animal studies have not identified an effect of arsenic on fertility in males or females. When females were dosed chronically for periods that included pregnancy, the primary effect of arsenic on reproduction was a dose-dependent increase in conceptus mortality and in postnatal growth retardation. Human data are limited to a few studies of populations exposed to arsenic from drinking water or from working at or living near smelters. Associations with spontaneous abortion and stillbirth have been reported in more than one of these studies, but interpretation of these studies is complicated because study populations were exposed to multiple chemicals. Thus, animal studies suggest that environmental arsenic exposures are primarily a risk to the developing fetus. In order to understand the implications for humans, attention must be given to comparative pharmacokinetics and metabolism, likely exposure scenarios, possible mechanisms of action, and the potential role of arsenic as an essential nutrient.