Ethinyloestradiol (10–50 μg) fed 1–4 days before mating delays first cleavage in up to 50 % of fertilized hamster eggs. This delay was not attributable to altered mating behaviour and fertilization appeared uneffected in ova recovered from treated animals one day after mating. It is suggested that early (possibly intrafollicular) development of the egg may be affected by this steroid, since it exerts an effect when given one day prior to mating, but appears to be without effect when administered on the day of mating.