Intravaginal administration to ovariectomized mice of 2·5 μg of either 3,3′,5,5′,α,α′-hexamethylstilboestrol (structure V) or 3,3′,5,5′-tetramethyl-α,α′-diethylstilboestrol (structure VI) significantly depressed the retention by vaginal tissue of concomitantly administered [6,7-3H]oestradiol-17β (0·75 × 10−4 μg), but did not affect the absorption of oestradiol from the vaginal lumen. The antagonism was shown by subcellular fractionation studies on vaginal homogenates to occur in the nuclear fraction.