Preimplantation mouse embryos were exposed to ricin, a plant toxin, in vitro and in vivo. The effect was evaluated by morphological observations of the exposed embryos as well as by means of protein synthesis. Ricin was highly toxic to the preimplantation mouse embryo in vitro, the effect being greater on the 2‐cell than on the 4–8‐cell stage. Intraperitoneal injection of ricin induced a small number of fetuses with exencephaly.