In the mouse uterus the development of sensitivity to a decidualizing stimulus requires large amounts of progesterone and small amounts of oestradiol-17 beta. During the process progesterone induces changes in the sensitivity of the tissues of the uterus to oestrogen. From observations of human endometrium it has been suggested that progesterone modulates the biological activity of oestradiol-17 beta by stimulating the metabolic conversion of oestradiol-17 beta to oestrone. Accordingly [6,7-3H]oestradiol-17 beta was injected subcutaneously into ovariectomized mice at various stages of the development of sensitivity to a decidualizing injection of oil into the uterine lumen. Radioactivity was extracted 4 h later, fractionated and identified. There was no alteration in the amounts of oestradiol, oestrone or water-soluble metabolites in the uteri whether the mice were treated with progesterone or with progesterone plus oestrogen or whether the uterine horns were decidualized for 24 or 48 h. The results suggest that in vivo the metabolism of oestradiol-17 beta by the uterus is not stimulated by progesterone.