To elucidate what kind of medium would be desirable for mammalian embryo culture, we evaluated the effect of heavy metal ions on mouse in vitro embryonic development. Pronuclear stage embryos recovered from ICR mice were cultured in Ham's F-10 and α-MEM and the developmental ability was compared between them. The major difference of these 2 media is that only Ham's F-10 contains heavy metal ions such as Zn²⁺, Fe²⁺ and Cu²⁺, and hypoxanthine. When pronuclear stage embryos were cultured in α-MEM, the rates of embryos reaching the 4-cell, blastocyst and hatched blastocyst stages were 96.5%, 75.4% and 66.7%, respectively. These values were significantly (P<0.01) higher than the rates of embryos cultured in Ham's F-10; 61.8%, 5.5% and 3.6% respectively. The deletion of CuSO₄, ZnSO₄, FeSO₄ and hypoxanthine from Ham's F-10 significantly increased the rates of embryos reaching the 4-cell, blastocyst and hatched blastocyst stages to the extent comparable to those in α-MEM. In contrast, the addition of all or one of CuSO₄, ZnSO₄, FeSO₄ and/or hypoxanthine to α-MEM significantly decreased the in vitro embryonic development. The strongest inhibition was observed when all of them were added. The developmental ability in α-MEM to which all of them were added was as low as that in Ham's F-10. These results suggest that the low developmental ability in Ham's F-10 may be mainly due to the deleterious effect of heavy metal ions and hypoxanthine. The toxic effect of heavy metal ions and hypoxanthine might be interpreted as the damage on embryos by an increased generation of oxygen radicals and the medium without constituents which may enhance the production of oxygen radicals seems to be desirable for the culture of mammalian embryos.