In the mouse, estrogen is essential for blastocyst implantation in the progesterone (P₄)-primed uterus. The mechanism(s) by which estrogen initiates this response still remains elusive. The present investigation, using delayed implantation in the mouse, examined the differential role of estradiol-17β (E₂) and its catechol metabolite 4-hydroxy-E₂ (4-OH-E₂) in uterine and blastocyst activation for implantation. The conditions of delayed implantation were induced by ovariectomizing mice on day 4 (day 1 = vaginal plug) of pregnancy or pseudopregnancy and maintaining them with P₄ from days 5–7. The binding of EGF to blastocysts was used as a marker for blastocyst activation. Our results show that whereas E₂ fails to activate dormant blastocysts (with respect to EGF binding in vitro), 4-OH-E₂, cAMP, or prostaglandin E₂, is effective in this response. Further, whereas 4-OH-E₂ induced-activation is not blocked by an antiestrogen, an inhibitor of PG synthesis, adenylyl cyclase or protein kinase A effectively blocks this activation. These results suggest that 4-OH-E₂ effects on blastocysts are mediated by PGs, which, in turn, stimulate cAMP production and thus activation of protein kinase A. Two-fluoro-E₂ is a poor substrate and an inhibitor of catecholestrogen synthesis, but it is estrogenic, with respect to uterine growth and gene expression. Using blastocyst transfer experiments, we observed that dormant blastocysts incubated with 4-OH-E₂ in vitro, but not with E₂, are capable of implanting in P₄-treated delayed implanting mice receiving two-fluoro-E₂. The results suggest that whereas E₂ is necessary for preparation of the uterus, uterine-derived catecholestrogen is important for blastocyst activation for implantation. Indeed, the receptive uterus has the capacity to synthesize 4-OH-E₂. Collectively, we demonstrate that the primary ovarian estrogen E₂, via its interaction with nuclear estrogen receptors, participates in the preparation of the P₄-primed uterus to the receptive state in an endocrine manner, whereas its metabolite 4-OH-E₂, produced from E₂ in the uterus, mediates blastocyst activation for implantation in a paracrine manner. Our results also establish that these target-specific effects of primary estrogen and catecholestrogen are both essential for implantation and that successful implantation occurs only when the activated stage of the blastocyst coincides with the receptive state of the uterus.