Day-11 spherical and Day-13 filamentous blastocysts were cultured in vitro without supplementation of oestrogen precursors for 2 consecutive periods of 24 h, with or without endometrial tissue. Spherical Day-11 blastocysts flushed from one uterine horn (5.5 /- 0.8 blastocysts; 5.3 /- 0.4 mm in diameter; mean /- s.d.) did not show gross morphological changes during the culture period. Free and conjugated oestrone (E1) and oestradiol-17 beta (E2) were released during the first and second 24 h culture period and was maximal during the first 24 h. In the presence of endometrial tissue the release of oestrogens from the blastocysts was reduced to 30-50%. Clewed Day-13 filamentous blastocysts, recovered after flushing, formed trophospheres and also in these cultures both free and conjugated E1 and E2 were released. In this case the addition of endometrial tissue did not affect the release of oestrogens from the blastocysts. Single Day-13 filamentous blastocysts cultured without endometrial tissue (n = 33, from 4 gilts), developed trophospheres and all blastocysts released E1 and E2. During the first 24 h on average per blastocyst 2.6 ng E1 (range 1.6-4.0) and 9.7 ng E2 (range 4.3-15.3) were released. In all cultures of spherical and filamentous blastocysts E2 release exceeded E1 release.These data show that Day 11-13 blastocysts are able to release both free and conjugated E1 and E2 during in vitro culture from endogenous precursors. The addition of endometrial tissue reduced the release of oestrogens from Day-11 but not Day-13 blastocysts. Whether this is due to an altered oestrogen release of the blastocysts in the presence of endometrium on Day-11 or due, to an increased uptake of oestrogens by the endometrium on Day-11 remains to be investigated.