Intraluminal injection of the anti-oestrogen, CI-628, into one uterine horn of rabbits on Day 5 of pregnancy inhibited implantation compared to the control horn. Exposure of 5-day blastocysts to 5×10−6 m-CI-628 in vitro prevented their implantation after transfer to synchronized pseudopregnant recipients. Incubation of blastocysts with CI-628 followed by further incubation in inhibitor-free medium significantly restored the proportion of implantations. Binding of [3H]oestradiol to Day-5 or Day-6 blastocysts was reduced by CI-628 and sucrose density gradient fractionation of blastocyst cytosol, incubated with [3H]oestradiol, revealed binding of radioactivity to soluble protein which was displaced in the presence of the inhibitor. The results implicate oestradiol in the development or implantation of the rabbit blastocyst and are consistent with the hypothesis that this steroid may act as a local signal from the blastocyst to the uterus.