The uptake of [6,7-3H]oestradiol-17\g=b\(oestradiol) by the rat uterus in vivo has been studied. By pressing uteri between two glass plates much, but not all, of the epithelium is removed with minimal effect on the underlying stroma and myometrium. This procedure removes 11 % of the DNA, 10\m=.\6% of the radioactivity and about 3 % of the wet weight of the intact uterus, indicating that, as judged from the DNA content, there is no accumulation of [6,7-3H]oestradiol in the epithelium relative to the rest of the uterus. In the epithelium, the major portion of the [6,7-3H]oestradiol was in the soluble fraction although a considerable amount of the radioactivity was associated with the nuclei. Similar results were obtained with nuclei isolated by differential or gradient centrifugation. It has been shown that an interchange of [6,7-3H]oestradiol can occur between the nucleus and cytoplasm and that, under the experimental conditions used, the equilibrium favours the cytoplasm.Some of the [6,7-3H]oestradiol in the soluble fraction from uterine epithelium was associated with a component of high molecular weight that may be a protein.