SUMMARY: Dimethylstilboestrol, a pro-oestrogen, inhibits the response to oestrone, oestradiol or diethylstilboestrol when both are given intravaginally to spayed mice by a 2-injection technique. It inhibits the natural oestrogens in doses of 0·05–0·5% of its own median effective dose (ca. 50 μg), but 5% of the median effective dose is needed with diethylstilboestrol. No inhibition was seen in the present tests when either oestradiol (used in most tests) or inhibitor were given subcutaneously, or when both were so given. Little or no inhibition was seen when oestradiol and inhibitor were given other than simultaneously. It is suggested that the untransformed molecule is the active inhibitor.