SUMMARY: [3H]Oestradiol-17β binding by cytoplasmic and nuclear fractions of human uterine tissue homogenates was studied by intact tissue and cell-free incubation techniques. In the absence of divalent cations, swelling and rupture of endometrial nuclei led to a loss of nuclear receptor complex into the cytoplasmic fraction of the homogenate. Divalent cations at a concentration of 0·003 mol/l, however, caused precipitation of the cytoplasmic receptor complex, inhibited [3H]oestradiol-17β binding by the cytoplasmic receptor and inhibited the extraction of soluble nuclear receptor complex. These effects were minimal when 0·001 m-magnesium ions were added to the sucrose—tris buffer. Studies of the distribution of oestrogen receptors in homogenates of human uterine tissue in 0·001 m-magnesium—sucrose—tris buffer revealed the presence of a soluble and an insoluble nuclear receptor complex. More of the nuclear receptor complex was obtained in the soluble form when the incubation temperature was raised from 25 to 37 °C. Evidence is presented for the transfer of [3H]oestradiol-17β from the cytoplasmic to the nuclear fractions by a temperature-dependent process and for the binding of [3H]oestradiol-17β by nuclear receptors in the absence of the cytoplasmic fraction.