Actin mRNA levels were measured in mouse eggs, early embryos, and delayed implanting blastocysts by a homologous, cloned recombinant DNA probe and "dot" blot methodology. A maternal store of 431 fg of actin mRNA was observed in the unfertilized eggs. This mRNA pool decreased 12-fold by the mid-two-cell stage. Actin mRNA levels were then observed to increase progressively from the eight-cell to the blastocyst stage on a basis proportional to cell number. Late blastocysts contained 2400 fg actin mRNA per embryo (22 fg per cell). The cellular level decreased by about 20% in embryos induced into delay of implantation by ovariectomy of donor females. Reactivation of the delayed implanting blastocysts through hormonal manipulation in vivo or culture in vitro was accompanied by reestablishment of the level of cellular actin mRNA observed in normal blastocysts.