The anti-oestrogenic and oestrogen-like potencies of clomiphene citrate are presumably mainly responsible for the mode of action of this synthetic compound in different organs of the female endocrine system. The investigations described in this paper should give more informations about these properties of clomiphene. Therefore the in vivo effect of cis- or trans-clomiphene on some metabolic pathways of the uterine tissue was tested. Studying the influence of cis- or trans-clomiphene on the 3H-oestradiol uptake into uterine tissue in vivo, the cis-isomer indicated the higher anti-oestrogenic effectiveness. In contrast to this observation both isomers were able to imitate biochemical effects of natural oestrogens in the uterus, i.e. the stimulation of the protein- and RNA-synthesizing activity of this organ. These biochemical alterations however, appeared some hours later as compared with 17β-oestradiol. On the base of these results and of experimental data obtained from the literature a model for the mode of action of clomiphene in the female reproductive system is proposed