Blastocysts were recovered from mice with experimentally produced delay of implantation. The dormant embryos were incubated in vitro for up to 24 h in medium containing [3H]uridine and supplemented with mouse serum or bovine serum albumin. Outgrowth of the trophoblast cells occurred in the presence of serum but not with bovine serum albumin. In contrast, the rate of incorporation of [3H]uridine into RNA by the embryos increased steadily throughout the period of incubation and was not influenced by the presence of serum. The change in incorporation of [3H]uridine was due to an increase in the overall rate of RNA synthesis and serum therefore has not effect on this aspect of embryo activation. The observation that a stimulatory serum factor is necessary for outgrowth of dormant embryos in vitro, but is not required for increased metabolic activity, indicates that these two aspects of embryo activation are regulated differently in vivo. With the assumption that trophoblast outgrowth and the changes in metabolic activity in vitro are analogous to the events that occur when embryonic diapause is terminated in vivo, it is suggested that the process of embryo activation after delayed implantation proceeds in a stepwise fashion with each phase being controlled in different ways.