In mammalian development, the embryo which is in the process of compaction, involves a progressive flattening of blastomeres against each other with the consequence that the embryo assumes a spherical shape. This stage happens in the first differentiation. The present study was aimed to examine the involvement of other metal ions in compaction by treating with various divalent cations in the absence of $Ca^{++}$. When 8-cell embryos were cultured in $Ca^{++}$-free medium for 24hrs, they developed to 16-cell stage but did not compaction, and degenerated after 48hrs of culture. Embryos were cultured in $Ca^{++}$-free medium for 24hrs and then transferred to the control medium showed the normal compaction afterwards. When 8-cell embryos were cultured in the presence of $Ni^{++}$, known as a $Ca^{++}$ inhibitor, they cleaved to 16-cell stage but did not compact in the absence of $Ca^{++}$. On the other hand, embryos cultured in the media containing both $Ca^{++}$ and $Ni^{++}$ developed normally so that they underwent compaction during culture for 48hrs. However, they failed to hatch during further 24hrs in the same medium, indicationg that $Ni^{++}$ may exert some harmful effects. Embryos grow in the control medium that contained $Ca^{++}$ but not $Ni^{++}$, developed to the hatched blastocysts. The treatment with $Cd^{++}$ $10^{-1}$,$10^{-2}{mu}M$, $Mn^{++}$ or $Ba^{++}$ 10,100, $1000{mu}M$ in $Ca^{++}$-free medium, respectively, inhibited compaction and embryonic degeneration began as in $Ca^{++}$-free medium. When 3, 5, 10mM of $Sr^{++}$, known as a substitute for $Ca^{++}$ in cell, was added to $Ca^{++}$-free medium, respectively, compaction was induced unlike the above metal ions. Embryos were cultured in $Sr^{++}$ developed to blastocysts, but failed to hatch after 72hrs and degenrated. On the other hand, when embryos were cultured in 3, 5, 10mM of $Sr^{++}$ but in $Ca^{++}$-free medium for 24hrs respectively and then transferred to the control, they showed the similiar development as that in the control.