SUMMARY The capacity of various oestrogens and anti-oestrogens to inhibit early pregnancy in the mouse has been compared. Both classes of compound are effective, but there remains some doubt as to the mode of action of the latter, since they are also oestrogenic (or pro-oestrogenic) in high doses. However, differences exist between the relative potency of dimethylstilboestrol (DMS, the most potent anti-oestrogen tested) and oestradiol as anti-fertility agents, and their relative potency as oestrogens. Differences also exist between their capacities to interrupt pregnancy in single doses. It is therefore concluded that DMS, at least, probably acts by virtue of its anti-oestrogenic activity.