Abstract A double tracer in vitro perfusion method has been used to estimate rates of entry, release and interconvertion of oestrone and oestradiol-17β in liver slices from immature and adult mice of both sexes. Uptake and release of the hormones were not profoundly altered by the age and sex of the experimental animals. The rate of release of oestrone from the tissue was 2–10 times higher then the release of oestradiol-17β. This difference was most marked in the immature animals. Extensive interconvertion of oestrone and oestradiol-17β was observed. In the adult female and in immature animals fractional convertion of oestradiol17β into oestrone was about twice that of the opposite reaction. In the adult males this pattern was almost exactly reversed. Consequently, in the adult male group, oestradiol-17β was found to be the sole precursor for further metabolism of oestrogens, while in the immature animals only oestrone served this purpose. Adult females showed an intermediate pattern, although oestrone was the main precursor.