Day 6 rabbit blastocysts that had previously been incubated with 10 nM [3H]prostaglandin E2 (PGE2), [3H] PGF2 alpha, or 0.5 microCi [3H]water were surgically transferred to the uteri of day 6 pseudopregnant recipient rabbits. At 1, 3, and 20 h after transfer, blastocysts were collected from the recipient rabbits and evaluated for retention of [3H]ligands. The transferred day 6 blastocysts were able to retain a significant proportion of the initial PGs for up to 20 h in vivo. These PGs were not metabolized while they remained in the blastocysts in vivo, as assessed by HPLC. The transferred blastocysts appeared normal and viable after 20 h in vivo, since implantation sites could be detected visually. These data support our hypothesis that blastocysts can sequester PGs from their environment in vivo and retain them unmetabolized during the critical period just before implantation.