AbstractZinc, at a concentration of 5 or 10 μg/ml medium, has been shown to protect mouse preembryos in vitro from the toxicity of cadmium at a concentration of 5 μg/ml medium after a simultaneous treatment of the ions from four‐cell to morula or from morula to blastocyst for 24 hours. Such an amelioration indicates that cadmium toxicity is a result of the unique property of the cadmium ion and is not due to an alteration in the culture medium after the addition of an extra metallic ion. Zinc probably ameliorates cadmium treated mouse preembryo by competing with cadmium for uptake or some other metabolic processes. In view of the well‐documented competition between cadmium and zinc ions for binding sites in many other cell types, some macromolecules to which similar divalent metallic ions bind competitively may also exist in the mouse preembryo. This suggests that a protective mechanism dependent on the metal‐metal interactions begins to operate in the mouse preembryo at a very early stage of differentiation before impla