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The activity of the 17β-hydroxysteroid oxidoreductase (17β-HSOR), catalysing the oxidoreduction of oestradiol-17β and oestrone, has been studied in the cytosol fraction of rat liver under various conditions. The activity of the enzyme increased during maturation and reached a plateau at 100 days in females and at 180 days in males. In adult male rats, the activity of the 17β-HSOR was about 60% higher than in adult female rats. When female animals were castrated, the development of enzyme activity was similar to that observed in male rats; there was no difference in enzyme activity between adult castrated female rats and normal male rats. In normal female rats the activity of the 17β-HSOR was high during metoestrus and dioestrus, and low during pro-oestrus and oestrus. These findings show that oestrogens have a repressing effect on the activity of a key enzyme of steroid metabolism in rat liver.