Abstract A mixture of cortisol- 3H and cortisone- 14C was administered to guinea pig foetuses in situ at mid-term and full-term gestation and to adult guinea pigs. After 30 min the radioactivity recovered in the liver accounted for 3–11% of the administered dose. In the foetal liver at midterm a significant percentage of cortisol was converted to cortisone, but the reverse conversion did not occur. The latter conversion took place to a small extent at the end of gestation. In the liver of the adult animals conversion occurred in both directions. In the foetal liver the greater part of the radioactivity was unconjugated. On the other hand, in adult liver tissues conjugated radioactive material accounted for 27–30%. Five to fourteen per cent of the total radioactivity in the hepatic cells was associated with the nuclear fraction and 60–80% of the tritiated radio-active material in this fraction consisted of unchanged cortisol; unchanged cortisone accounted for 30–40%.