An interaction between the blastocyst and the uterus is essential for establishment of pregnancy. Because maternal estrogen is not an absolute requirement, estrogen of embryonic origin has been implicated in this process in the pig and the rabbit. Furthermore, estrogen forming capacity has been documented in the blastocyst of these species. However, while the complete machinery for steroid synthesis in the pig balstocyst has been demonstrated, the issue is still unresolved for the rabbit blastocyst. In the present communication we have shown that 17α-hydroxylase and C17–20-lyase, enzymes involved in the formation of androgens (C19-steroids) from C21-steroids (progestins), are present in day-6 rabbit blastocysts. C17–20-lyase activity was undetectable to low in day-5 and increased in day-6 balstocysts. The activity was further increased in day-6 blastocysts cultured for 24 h. Because prostaglandins have been implicated in uterine vascular changes at about the time of implantation and pregnancy establishment, and because catechol estrogens are more potent than phenolic estrogens in stimulating prostaglandin synthesis in the blastocyst and the uterus, we determined catechol estrogen forming capacity in the rabbit and pig blastocyst. Catechol estrogen forming capacity (estrogen-2/4-hydroxylase) in the pig blastocyst appears on day 10 of pregnancy, peaks on day 12 and then declines. Our preliminary experiments also indicate that day-6 rabbit blastocysts have catechol estrogen forming capacity. On the basis of our present findings and of others, we propose that catechol estrogens of embryonic origin mediate the stimulatory effect of estrogens on prostaglandin synthesis in the embryo and/or the uterus and thus participate in the process of establishment of pregnancy.