Summary. In-vitro treatment of preimplantation mouse embryos with spermine and spermidine biosynthesis inhibitor, methylglyoxal-bis-(guanylhydrazone) (MGBG), arrested embryo development at the 8-cell or morula stage. In addition, the embryo DNA synthetic rate, as measured by [3H]thymidine incorporation, was strongly inhibited. The inhibition of blastocyst formation and DNA synthesis by MGBG was readily reversible by an exogenous supply of spermine and/or spermidine to the culture medium. dl-α-Methylornithine or dl-α-difluoromethylornithine (α-DFMO), inhibitors of putrescine biosynthesis, had no effect on embryos cultured for 1 or 2 days, but on the 3rd day embryo DNA synthesis was significantly depressed in the presence of α-DFMO. These observations suggest that, during early development of the pre-implantation mouse embryo, spermine and spermidine are involved in regulation of embryo growth and DNA synthesis. They may also indicate a role of putrescine at a later stage of mouse embryo development.