The subnuclear distribution of 3H-oestradiol-receptor complexes was studied in uterine tissue of ovariectomized adult rats. Nuclei were sonically disrupted and 8 different subnuclear fractions were isolated by discontinuous sucrose density gradient centrifugation. 3H-Oestradiol-receptor complexes, measured by hydroxylapatite column chromatography, were localized in a light chromatin fraction as well as in a heavy chromatin fraction. Using the hydroxylapatite chromatography technique it was possible to demonstrate three classes of oestradiol-receptor complexes which differ in affinity for the chromatin. Oestradiol-receptor complexes with a high affinity for the chromatin were predominantly localized in the heavy chromatin fraction, whereas complexes with a lower affinity for their acceptor sites were present in the lighter chromatin fraction.