Oestradiol and progesterone receptors concentration in endometrium is regulated by oestradiol and progesterone themselves. Oestradiol induces the two types of receptors, mainly the progesterone receptor. Progesterone decreases the production of the two types of receptors and probably increases the turnover of its own receptor. Pharmacological agents, synthetic oestrogen and progestins, behave similarly. The antioestrogen tamoxifen has a partial agonistic action of the oestrogen type on progesterone receptor induction. These observations, however, concern the whole tissue. At the cellular level, the regulation of the production of the two receptor types may indeed be different from what is observed in the entire tissue. Finally recent data on the presence of specific binding sites for antioestrogens and on receptor heterogeneity justify new studies of these regulating factors.