Embryotoxic effects of two inorganic arsenic compounds, sodium arsenite (Asi) and sodium arsenate (Asa), on the development of mouse embryos during early organogenesis were studied using the whole embryo culture technique. Embryos with three to five somites exposed to 1–40 μM Asi or to 10–400 μM Asa were cultured for 48 hours and their development was compared with that of control embryos. Asi proved to be teratogenic between 3 and 4 μM and embryolethal at higher concentrations; Asa had similar activity but at concentrations ten times higher than for Asi. Both compounds produced a growth retardation and a similar pattern of defects. Growth retardation was indicated by a statistically significant reduction in crown‐rump length, head length, and yolk sac diameter. Abnormal embryos were characterized by hypoplasia of the prosencephalon with open neural tube, hydropericardium, somite abnormalities, and failure of development of limb buds and sensory placodes. These results confirm that both Asa and Asi are embryotoxic compounds and that the Asi activity occurs at concentrations ten times lower than for Asa. Our results suggest that in humans both of these compounds may be involved in part of “unaccountable” early abortions and malformations claimed to be due to the toxicity of heavy metals.