1. Plasma corticosterone concentrations were determined in the foetal rat during the gestational period from day 18·5 to term and in postnatal rats over the first few hours after delivery. 2. The plasma corticosterone concentrations in foetal rats are as high as six times maternal values at day 19 of gestation and are approximately equal to maternal values from day 20 to term. 3. In postnatal rats the plasma corticosterone concentrations rise 3·5-fold on average within 5hr. of delivery. 4. The results are discussed in relation to the function of adrenal steroids in postnatal liver development.
