It was demonstrated by the present study that glycyrrhizin inhibited the action of estradiol-17β on uterine weight and β-glucuronidase activity in ovariectomized mice and rats in a certain dose ratio. The inhibition was 50% or more when the dose ratio of glycyrrhizin: estradiol-17β was 1000: 1. In any higher ratio of glycyrrhizin to estrogen, the inhibition was decreased or even reversed. Glycyrrhizin by itself had no effect on either uterine weight or β-glucuronidase activity in ovariectomized mice. The mechanism of the anti-estrogenic effect was discussed in relation to the effect of glycyrrhizin on biological actions and hepatic degradation of cortisone.