After oestradiol administration in vivo, 87-95% of the initial concentration of oestradiol receptor in the cytoplasm of the embryonic-chick Müllerian-duct cell was translocated into the nucleus. The process of translocation depends on the amount of oestardiol administered in vivo. At 6 h after oestradiol administration in vivo, about 30% replenishment of the initial content of the cytosol receptor was observed in the cytoplasm. The Müllerian-duct nuclei, after exposure to non-radioactive oestradiol, exhibit saturable exchange with [3H]oestradiol in vitro. The exchange of oestradiol is temperature- and time-dependent. The optimal temperature and time for exchange are 37-41 degrees C and 2h respectively. The [3H]oestradiol-receptor complex extracted from the exchanged nuclei is present in 5-6S form, and its isoelectric point is 6.8. The number of nuclear oestradiol-binding sites of the developing Müllerian duct are 1.66, 2.22, 2.63, and 2.50 pmol/mg of DNA respectively for embryos of 10, 12, 15 and 18 days. The dissociation constants of the nuclear oestradiol receptor of the four observed developmental stages range from 3.0 to 3.1 nM.