Summary. Mouse embryos (4–8 cells) were cultured for 48 h in vitro in medium containing 1,4,6-androstatriene-3,17-dione (ATD). The inhibition of embryo development by ATD was overcome by oestradiol-17β but not by oestradiol-17α. Culture with ATD for the first and second 24 h led to normal numbers of blastocysts. The results show that preimplantation mouse embryos contain the enzymic machinery for synthesis of oestrogen.