The effect of progesterone and oestradiol on blastocysts cultured within the lumina of immature mouse uteri.

1. Ovariectomy-delayed blastocysts flushed from mice treated with medroxyprogesterone acetate were injected into the lumina of immature mouse uterine horns. The horns were then maintained as organ cultures for 1-3 days in media that contained either (a) ethanol or ethanol plus (6) progesterone, (c) oestradiol or (d) progesterone and oestradiol. 2. Trophoblast invasion of the uterine stroma occurred for 13, 10 and 6 % of the eggs injected, in media that contained progesterone, progesterone plus oestradiol, and oestradiol, respectively. The uterine stromal tissue did not decidualize. 3. Embryonic development in invading eggs usually became abnormal during the migration of the distal endoderm. Further development was impeded by the progressive collapse of the yolk cavity which allowed trophoblastic giant cells to intermingle with proximal endodermal cells. 4. The optimal oxygen concentration for trophoblastic invasion and embryonic development in the presence of progesterone and oestradiol was between 26 and 40 %.