Oestrone sulphatase activity was determined in the 12,500 g supernatant of rat uterus homogenates. The conditions used were 0.5 mM oestrone sulphate and 30 min incubation at pH 8.0 and 37 °C. The enzyme activity in uteri from normal animals was found to be 5.1 ± 0.7 nmol (mg protein)<sup>-1</sup>h-<sup>1</sup>. In ovariectomized and hypophysectomized animals the values were significantly higher, whereas subcutaneous administration of 17 β-oestradiol to such rats resulted in significantly decreased oestrone sulphatase activities. Progesterone increased the enzyme activity of intact rats. It is concluded that the uterine tissue of rats contains oestrone sulphatase which may be of biological importance and which is inhibited by oestrogen.