SUMMARY The effect of age and the role of the pituitary and gonads on the change in liver oestrogen hydroxylase activity induced by oestrone and testosterone has been investigated. Testosterone increased the hydroxylation of oestradiol in immature rats but had no effect in adult female animals unless they had been ovariectomized. In male rats, hypophysectomy resulted in a rapid decrease in oestrogen metabolism which could not be reversed by treatment with testosterone. Oestrone produced reversible changes in oestrogen hydroxylation in adult male rats in contrast to the irreversible effects observed in immature animals. The significance of these results in the control of oestrogen metabolism is discussed.