The phyto-oestrogens coumestrol (7,12-dihydroxycoumestan) and genistein (5,7,4′-trihydroxyisoflavone) induced uterine growth in ovariectomized rats, but failed to induce ovum implantation in mated ovariectomized, gestagen-maintained rats and in intact lactating rats when given at dose levels effective in uterine weight assays. Oestradiol17β, under similar conditions, induced ovum implantation even at dose levels below those required for a uterine weight response. Diethylstilboestrol and large doses of oestradiol-17α (5 to 10 μg) were effective in both assays. It is suggested that different uterine receptors may be involved in mediating the uterine-growth-promoting and implantationinducing actions of oestrogen.